背景介紹
多用途互聯網郵件擴展，它是一個互聯網标準，在1992年最早應用于電子郵件系統，但後來也應用到浏覽器。服務器會将它們發送的多媒體數據的類型告訴浏覽器，而通知手段就是說明該多媒體數據的MIME類型，從而讓浏覽器知道接收到的信息哪些是MP3文件，哪些是Shockwave文件等等。服務器将MIME标志符放入傳送的數據中來告訴浏覽器使用哪種插件讀取相關文件。
MIME能夠支持非ASCII字符、二進制格式附件等多種格式的郵件消息。這個标準被定義在RFC 2045，RFC 2046，RFC 2047，RFC 2048，RFC 2049等RFC中。 由RFC 822轉變而來的RFC 2822，規定電子郵件标準并不允許在郵件消息中使用7位ASCII字符集以外的字符。正因如此，一些非英語字符消息和二進制文件，圖像，聲音等非文字消息都不能在電子郵件中傳輸。MIME規定了用于表示各種各樣的數據類型的符号化方法。
浏覽器接收到文件後，會進入插件系統進行查找，查找出哪種插件可以識别讀取接收到的文件。如果浏覽器不清楚調用哪種插件系統，它可能會告訴用戶缺少某插件，或者直接選擇某現有插件來試圖讀取接收到的文件，或者可能會導緻系統的崩潰。傳輸的信息中缺少MIME标識可能導緻的情況很難估計，因為某些計算機系統可能不會出現什麼故障，但某些計算機可能就會因此而崩潰。
檢查一個服務器是否正确設置了MIME類型的步驟是：
在Netscape浏覽器中打開服務器網頁。
進入"Tools"菜單，選擇"Page Info"。
在彈出的窗口中點擊上層框架中的"EMBED"。
在下層框架中查看MIME的類型是否為"application/x-director"或"application/x-shockwave-flash"，如果是上述信息的話表明服務器已經正确設置了MIME類型；而如果MIME類型列出的是文本内容、八位一組的數據或是其它形式均表明服務器的MIME類型沒有設置正确。
如果服務器沒有正确标明其發送的數據的類型，服務器管理員應該正确添加相關信息，具體操作方法非常簡單快捷。
Microsoft公司應用于Windows系統下的浏覽器使用ActiveX控件，而不是Netscape插件，這種浏覽器不必象其它浏覽器那樣依靠MIME的編碼。"OBJECT"标簽的"CLSID"屬性準确地标明了應調用哪種程序來讀取接收到的文件，因此浏覽器不必象"EMBED"标簽那樣選擇一種讀取程序。正因為如此，你往往會在使用帶插件的浏覽器時遇到MIME問題，而使用ActiveX控件的浏覽器則很少出現此類麻煩。
正由于上述工作方式的差别也解釋了一種現象，不知你是否發現在使用Netscape浏覽器播放WAV文件時，浏覽器會調用LiveConnect插件進行播放，而其它浏覽器一般都使用通用的QuickTime的播放插件等來進行播放，這是因為Netscape浏覽器接收文件需要讀取MIME标識符，以便決定調用哪種程序來讀取接收的文件，而服務器設置在Netscape浏覽器中播放WAV文件應使用LiveConnect插件，因此Netscape浏覽器接收到WAV格式的文件時必然就會調用LiveConnect插件，但由于其它浏覽器不使用這種方式，因此它們都使用系統默認的播放WAV格式文件的播放器。當然Flash電影文件并不存在這種問題，因為隻有Flash播放器才能夠正确讀取這種格式的文件。
在把輸出結果傳送到浏覽器上的時候，浏覽器必須啟動适當的應用程序來處理這個輸出文檔。這可以通過多種類型MIME（多功能網際郵件擴充協議）來完成。在HTTP中，MIME類型被定義在Content-Type header中。
例如，假設你要傳送一個Microsoft Excel文件到客戶端。那麼這時的MIME類型就是“excel”。在大多數實際情況中，這個文件然後将傳送給Excel來處理（假設我們設定Excel為處理特殊MIME類型的應用程序）。在ASP中，設定MIME類型的方法是通過Response對象的ContentType屬性。
文件格式
最早的HTTP協議中，并沒有附加的數據類型信息，所有傳送的數據都被客戶程序解釋為超文本标記語言HTML 文檔，而為了支持多媒體數據類型，HTTP協議中就使用了附加在文檔之前的MIME數據類型信息來标識數據類型。
MIME意為多功能Internet郵件擴展，它設計的最初目的是為了在發送電子郵件時附加多媒體數據，讓郵件客戶程序能根據其類型進行處理。然而當它被HTTP協議支持之後，它的意義就更為顯著了。它使得HTTP傳輸的不僅是普通的文本，而變得豐富多彩。
每個MIME類型由兩部分組成，前面是數據的大類别，例如聲音audio、圖象image等，後面定義具體的種類。
七種大類别：
video
image
application
text
audio
multipart
message
常見的MIME類型(通用型)：
超文本标記語言文本 .html text/html
xml文檔 .xml text/xml
XHTML文檔 .xhtml application/xhtml+xml
普通文本 .txt text/plain
RTF文本 .rtf application/rtf
PDF文檔 .pdf application/pdf
Microsoft Word文件 .word application/msword
PNG圖像 .png image/png
GIF圖形 .gif image/gif
JPEG圖形 .jpeg,.jpg image/jpeg
au聲音文件 .au audio/basic
MIDI音樂文件 mid,.midi audio/midi,audio/x-midi
RealAudio音樂文件 .ra, .ram audio/x-pn-realaudio
MPEG文件 .mpg,.mpeg video/mpeg
AVI文件 .avi video/x-msvideo
GZIP文件 .gz application/x-gzip
TAR文件 .tar application/x-tar
任意的二進制數據 application/octet-stream
Internet中有一個專門組織IANA來确認标準的MIME類型，但Internet發展的太快，很多應用程序等不及IANA來确認他們使用的MIME類型為标準類型。因此他們使用在類别中以x-開頭的方法标識這個類别還沒有成為标準，例如：x-gzip，x-tar等。事實上這些類型運用的很廣泛，已經成為了事實标準。隻要客戶機和服務器共同承認這個MIME類型，即使它是不标準的類型也沒有關系，客戶程序就能根據MIME類型，采用具體的處理手段來處理數據。而Web服務器和浏覽器（包括操作系統）中，缺省都設置了标準的和常見的MIME類型，隻有對于不常見的 MIME類型，才需要同時設置服務器和客戶浏覽器，以進行識别。
由于MIME類型與文檔的後綴相關，因此服務器使用文檔的後綴來區分不同文件的MIME類型，服務器中必須定義文檔後綴和MIME類型之間的對應關系。而客戶程序從服務器上接收數據的時候，它隻是從服務器接受數據流，并不了解文檔的名字，因此服務器必須使用附加信息來告訴客戶程序數據的MIME類型。服務器在發送真正的數據之前，就要先發送标志數據的MIME類型的信息，這個信息使用Content-type關鍵字進行定義，例如對于HTML文檔，服務器将首先發送以下兩行MIME标識信息,這個标識并不是真正的數據文件的一部分。
Content-type: text/html
注意，第二行為一個空行，這是必須的，使用這個空行的目的是将MIME信息與真正的數據内容分隔開。
MIME利用了一個事實就是，RFC 822在消息體的内容中做了一點限制：唯一的限制就是隻能使用簡單的ASCII文本。所以，MIME信息由正常的Internet文本郵件組成，文本郵件擁有一些特别的符合RFC 822的信息頭和格式化過的信息體（用ASCII 的子集來表示的附件）。這些MIME頭給出了一種在郵件中表示附件的特别的方法。
信息剖析
一個普通的文本郵件的信息包含一個頭（header）部分（To: From: Subject: 等等）和一個體（body）部分（Hello Mr.,等等）。在一個符合MIME的信息中，也包含一個信息頭并不奇怪，郵件的各個部分叫做MIME段，每段前也綴以一個特别的頭。MIME郵件隻是基于RFC 822郵件的一個擴展，然而它有着自己的RFC規範集。
頭字段
MIME頭根據在郵件包中的位置，大體上分為MIME信息頭和MIME段頭。（譯者：MIME信息頭指整個郵件的頭，而MIME段頭指每個MIME段的頭。）
MIME信息頭有：
MIME-Version:
這個頭提供了所用MIME的版本号。這個值習慣上為1.0。
Content-Type:
它定義了數據的類型，以便數據能被适當地處理。有效的類型有：text，image，audio，video，application，multipart和message。注意任何一個二進制附件都應該被叫做application/octet- stream。這個頭的一些用例為：image/jpg, application/mswork，multipart/mixed，這隻是很少的一部分。
Content-Transfer-Encoding:
這是所有頭中最重要的一個，因為它說明了對數據所執行的編碼方式，客戶/MUA 将用它對附件進行解碼。對于每個附件，可以使用7bit，8bit，binary ，quoted-printable，base64和custom中的一種編碼方式。7bit編碼是用在US ASCII字符集上的常用的一種編碼方式，也就是，保持它的原樣。8bit和binary編碼一般不用。對人類可讀的标準文本，如果傳輸要經過對格式有影響的網關時對其進行保護，可以使用quoted printable 。Base64是一種通用方法，在需要決定使用哪一種編碼方法時，它提供了一個不用費腦子的選擇；它通常用在二進制，非文本數據上。注意，任何非7bit 數據必須用一種模式編碼，這樣它就可以通過Internet郵件網關！
Content-ID:
如果Content-Type是message/external-body或multipart/alternative時，這個頭就有用了，它超出了本文的範圍。
Content-Description:
這是一個可選的頭。它是任何信息段内容的自由文本描述。描述必須使用us-ascii碼。
Content-Disposition:
一個試驗性的頭，它用于給客戶程序/MUA提供提示，來決定是否在行内顯示附件或作為單獨的附件。
MIME段頭（出現在實際的MIME附件部分的頭），除了MIME-Version頭，可以擁有以上任何頭字段。如果一個MIME頭是信息塊的一部分，它将作用于整個信息體。例如，如果Content-Transfer-Encoding顯示在信息（指整個信息）頭中，它應用于整個信息體，但是如果它顯示在一個MIME段裡，它"隻能"用于那個段中。
注意：其可以對自動對收到的郵件進行解密。
類型大全
格式前面為後辍名，後面為對應的MIME型（例如：rar application/x-rar-compressed 表示。RAR對應的是application/x-rar-compressed ）
{ ".323", "text/h323" },
{ ".3gp", "video/3gpp" },
{ ".aab", "application/x-authoware-bin" },
{ ".aam", "application/x-authoware-map" },
{ ".aas", "application/x-authoware-seg" },
{ ".acx", "application/internet-property-stream" },
{ ".ai", "application/postscript" },
{ ".aif", "audio/x-aiff" },
{ ".aifc", "audio/x-aiff" },
{ ".aiff", "audio/x-aiff" },
{ ".als", "audio/X-Alpha5" },
{ ".amc", "application/x-mpeg" },
{ ".ani", "application/octet-stream" },
{ ".apk", "application/vnd.android.package-archive" },
{ ".asc", "text/plain" },
{ ".asd", "application/astound" },
{ ".asf", "video/x-ms-asf" },
{ ".asn", "application/astound" },
{ ".asp", "application/x-asap" },
{ ".asr", "video/x-ms-asf" },
{ ".asx", "video/x-ms-asf" },
{ ".au", "audio/basic" },
{ ".avb", "application/octet-stream" },
{ ".avi", "video/x-msvideo" },
{ ".awb", "audio/amr-wb" },
{ ".axs", "application/olescript" },
{ ".bas", "text/plain" },
{ ".bcpio", "application/x-bcpio" },
{ ".bin ", "application/octet-stream" },
{ ".bld", "application/bld" },
{ ".bld2", "application/bld2" },
{ ".bmp", "image/bmp" },
{ ".bpk", "application/octet-stream" },
{ ".bz2", "application/x-bzip2" },
{ ".c", "text/plain" },
{ ".cal", "image/x-cals" },
{ ".cat", "application/vnd.ms-pkiseccat" },
{ ".ccn", "application/x-cnc" },
{ ".cco", "application/x-cocoa" },
{ ".cdf", "application/x-cdf" },
{ ".cer", "application/x-x509-ca-cert" },
{ ".cgi", "magnus-internal/cgi" },
{ ".chat", "application/x-chat" },
{ ".class", "application/octet-stream" },
{ ".clp", "application/x-msclip" },
{ ".cmx", "image/x-cmx" },
{ ".co", "application/x-cult3d-object" },
{ ".cod", "image/cis-cod" },
{ ".conf", "text/plain" },
{ ".cpio", "application/x-cpio" },
{ ".cpp", "text/plain" },
{ ".cpt", "application/mac-compactpro" },
{ ".crd", "application/x-mscardfile" },
{ ".crl", "application/pkix-crl" },
{ ".crt", "application/x-x509-ca-cert" },
{ ".csh", "application/x-csh" },
{ ".csm", "chemical/x-csml" },
{ ".csml", "chemical/x-csml" },
{ ".css", "text/css" },
{ ".cur", "application/octet-stream" },
{ ".dcm", "x-lml/x-evm" },
{ ".dcr", "application/x-director" },
{ ".dcx", "image/x-dcx" },
{ ".der", "application/x-x509-ca-cert" },
{ ".dhtml", "text/html" },
{ ".dir", "application/x-director" },
{ ".dll", "application/x-msdownload" },
{ ".dmg", "application/octet-stream" },
{ ".dms", "application/octet-stream" },
{ ".doc", "application/msword" },
{ ".docx", "application/vnd.openxmlformats-officedocument.wordprocessingml.document" },
{ ".dot", "application/msword" },
{ ".dvi", "application/x-dvi" },
{ ".dwf", "drawing/x-dwf" },
{ ".dwg", "application/x-autocad" },
{ ".dxf", "application/x-autocad" },
{ ".dxr", "application/x-director" },
{ ".ebk", "application/x-expandedbook" },
{ ".emb", "chemical/x-embl-dl-nucleotide" },
{ ".embl", "chemical/x-embl-dl-nucleotide" },
{ ".eps", "application/postscript" },
{ ".epub", "application/epub+zip" },
{ ".eri", "image/x-eri" },
{ ".es", "audio/echospeech" },
{ ".esl", "audio/echospeech" },
{ ".etc", "application/x-earthtime" },
{ ".etx", "text/x-setext" },
{ ".evm", "x-lml/x-evm" },
{ ".evy", "application/envoy" },
{ ".exe", "application/octet-stream" },
{ ".fh4", "image/x-freehand" },
{ ".fh5", "image/x-freehand" },
{ ".fhc", "image/x-freehand" },
{ ".fif", "application/fractals" },
{ ".flr", "x-world/x-vrml" },
{ ".flv", "flv-application/octet-stream" },
{ ".fm", "application/x-maker" },
{ ".fpx", "image/x-fpx" },
{ ".fvi", "video/isivideo" },
{ ".gau", "chemical/x-gaussian-input" },
{ ".gca", "application/x-gca-compressed" },
{ ".gdb", "x-lml/x-gdb" },
{ ".gif", "image/gif" },
{ ".gps", "application/x-gps" },
{ ".gtar", "application/x-gtar" },
{ ".gz", "application/x-gzip" },
{ ".h", "text/plain" },
{ ".hdf", "application/x-hdf" },
{ ".hdm", "text/x-hdml" },
{ ".hdml", "text/x-hdml" },
{ ".hlp", "application/winhlp" },
{ ".hqx", "application/mac-binhex40" },
{ ".hta", "application/hta" },
{ ".htc", "text/x-component" },
{ ".htm", "text/html" },
{ ".html", "text/html" },
{ ".hts", "text/html" },
{ ".htt", "text/webviewhtml" },
{ ".ice", "x-conference/x-cooltalk" },
{ ".ico", "image/x-icon" },
{ ".ief", "image/ief" },
{ ".ifm", "image/gif" },
{ ".ifs", "image/ifs" },
{ ".iii", "application/x-iphone" },
{ ".imy", "audio/melody" },
{ ".ins", "application/x-internet-signup" },
{ ".ips", "application/x-ipscript" },
{ ".ipx", "application/x-ipix" },
{ ".isp", "application/x-internet-signup" },
{ ".it", "audio/x-mod" },
{ ".itz", "audio/x-mod" },
{ ".ivr", "i-world/i-vrml" },
{ ".j2k", "image/j2k" },
{ ".jad", "text/vnd.sun.j2me.app-descriptor" },
{ ".jam", "application/x-jam" },
{ ".jar", "application/java-archive" },
{ ".java", "text/plain" },
{ ".jfif", "image/pipeg" },
{ ".jnlp", "application/x-java-jnlp-file" },
{ ".jpe", "image/jpeg" },
{ ".jpeg", "image/jpeg" },
{ ".jpg", "image/jpeg" },
{ ".jpz", "image/jpeg" },
{ ".js", "application/x-javascript" },
{ ".jwc", "application/jwc" },
{ ".kjx", "application/x-kjx" },
{ ".lak", "x-lml/x-lak" },
{ ".latex", "application/x-latex" },
{ ".lcc", "application/fastman" },
{ ".lcl", "application/x-digitalloca" },
{ ".lcr", "application/x-digitalloca" },
{ ".lgh", "application/lgh" },
{ ".lha", "application/octet-stream" },
{ ".lml", "x-lml/x-lml" },
{ ".lmlpack", "x-lml/x-lmlpack" },
{ ".log", "text/plain" },
{ ".lsf", "video/x-la-asf" },
{ ".lsx", "video/x-la-asf" },
{ ".lzh", "application/octet-stream" },
{ ".m13", "application/x-msmediaview" },
{ ".m14", "application/x-msmediaview" },
{ ".m15", "audio/x-mod" },
{ ".m3u", "audio/x-mpegurl" },
{ ".m3url", "audio/x-mpegurl" },
{ ".m4a", "audio/mp4a-latm" },
{ ".m4b", "audio/mp4a-latm" },
{ ".m4p", "audio/mp4a-latm" },
{ ".m4u", "video/vnd.mpegurl" },
{ ".m4v", "video/x-m4v" },
{ ".ma1", "audio/ma1" },
{ ".ma2", "audio/ma2" },
{ ".ma3", "audio/ma3" },
{ ".ma5", "audio/ma5" },
{ ".man", "application/x-troff-man" },
{ ".map", "magnus-internal/imagemap" },
{ ".mbd", "application/mbedlet" },
{ ".mct", "application/x-mascot" },
{ ".mdb", "application/x-msaccess" },
{ ".mdz", "audio/x-mod" },
{ ".me", "application/x-troff-me" },
{ ".mel", "text/x-vmel" },
{ ".mht", "message/rfc822" },
{ ".mhtml", "message/rfc822" },
{ ".mi", "application/x-mif" },
{ ".mid", "audio/mid" },
{ ".midi", "audio/midi" },
{ ".mif", "application/x-mif" },
{ ".mil", "image/x-cals" },
{ ".mio", "audio/x-mio" },
{ ".mmf", "application/x-skt-lbs" },
{ ".mng", "video/x-mng" },
{ ".mny", "application/x-msmoney" },
{ ".moc", "application/x-mocha" },
{ ".mocha", "application/x-mocha" },
{ ".mod", "audio/x-mod" },
{ ".mof", "application/x-yumekara" },
{ ".mol", "chemical/x-mdl-molfile" },
{ ".mop", "chemical/x-mopac-input" },
{ ".mov", "video/quicktime" },
{ ".movie", "video/x-sgi-movie" },
{ ".mp2", "video/mpeg" },
{ ".mp3", "audio/mpeg" },
{ ".mp4", "video/mp4" },
{ ".mpa", "video/mpeg" },
{ ".mpc", "application/vnd.mpohun.certificate" },
{ ".mpe", "video/mpeg" },
{ ".mpeg", "video/mpeg" },
{ ".mpg", "video/mpeg" },
{ ".mpg4", "video/mp4" },
{ ".mpga", "audio/mpeg" },
{ ".mpn", "application/vnd.mophun.application" },
{ ".mpp", "application/vnd.ms-project" },
{ ".mps", "application/x-mapserver" },
{ ".mpv2", "video/mpeg" },
{ ".mrl", "text/x-mrml" },
{ ".mrm", "application/x-mrm" },
{ ".ms", "application/x-troff-ms" },
{ ".msg", "application/vnd.ms-outlook" },
{ ".mts", "application/metastream" },
{ ".mtx", "application/metastream" },
{ ".mtz", "application/metastream" },
{ ".mvb", "application/x-msmediaview" },
{ ".mzv", "application/metastream" },
{ ".nar", "application/zip" },
{ ".nbmp", "image/nbmp" },
{ ".nc", "application/x-netcdf" },
{ ".ndb", "x-lml/x-ndb" },
{ ".ndwn", "application/ndwn" },
{ ".nif", "application/x-nif" },
{ ".nmz", "application/x-scream" },
{ ".nokia-op-logo", "image/vnd.nok-oplogo-color" },
{ ".npx", "application/x-netfpx" },
{ ".nsnd", "audio/nsnd" },
{ ".nva", "application/x-neva1" },
{ ".nws", "message/rfc822" },
{ ".oda", "application/oda" },
{ ".ogg", "audio/ogg" },
{ ".oom", "application/x-AtlasMate-Plugin" },
{ ".p10", "application/pkcs10" },
{ ".p12", "application/x-pkcs12" },
{ ".p7b", "application/x-pkcs7-certificates" },
{ ".p7c", "application/x-pkcs7-mime" },
{ ".p7m", "application/x-pkcs7-mime" },
{ ".p7r", "application/x-pkcs7-certreqresp" },
{ ".p7s", "application/x-pkcs7-signature" },
{ ".pac", "audio/x-pac" },
{ ".pae", "audio/x-epac" },
{ ".pan", "application/x-pan" },
{ ".pbm", "image/x-portable-bitmap" },
{ ".pcx", "image/x-pcx" },
{ ".pda", "image/x-pda" },
{ ".pdb", "chemical/x-pdb" },
{ ".pdf", "application/pdf" },
{ ".pfr", "application/font-tdpfr" },
{ ".pfx", "application/x-pkcs12" },
{ ".pgm", "image/x-portable-graymap" },
{ ".pict", "image/x-pict" },
{ ".pko", "application/ynd.ms-pkipko" },
{ ".pm", "application/x-perl" },
{ ".pma", "application/x-perfmon" },
{ ".pmc", "application/x-perfmon" },
{ ".pmd", "application/x-pmd" },
{ ".pml", "application/x-perfmon" },
{ ".pmr", "application/x-perfmon" },
{ ".pmw", "application/x-perfmon" },
{ ".png", "image/png" },
{ ".pnm", "image/x-portable-anymap" },
{ ".pnz", "image/png" },
{ ".pot,", "application/vnd.ms-powerpoint" },
{ ".ppm", "image/x-portable-pixmap" },
{ ".pps", "application/vnd.ms-powerpoint" },
{ ".ppt", "application/vnd.ms-powerpoint" },
{ ".pptx", "application/vnd.openxmlformats-officedocument.presentationml.presentation" },
{ ".pqf", "application/x-cprplayer" },
{ ".pqi", "application/cprplayer" },
{ ".prc", "application/x-prc" },
{ ".prf", "application/pics-rules" },
{ ".prop", "text/plain" },
{ ".proxy", "application/x-ns-proxy-autoconfig" },
{ ".ps", "application/postscript" },
{ ".ptlk", "application/listenup" },
{ ".pub", "application/x-mspublisher" },
{ ".pvx", "video/x-pv-pvx" },
{ ".qcp", "audio/vnd.qcelp" },
{ ".qt", "video/quicktime" },
{ ".qti", "image/x-quicktime" },
{ ".qtif", "image/x-quicktime" },
{ ".r3t", "text/vnd.rn-realtext3d" },
{ ".ra", "audio/x-pn-realaudio" },
{ ".ram", "audio/x-pn-realaudio" },
{ ".rar", "application/octet-stream" },
{ ".ras", "image/x-cmu-raster" },
{ ".rc", "text/plain" },
{ ".rdf", "application/rdf+xml" },
{ ".rf", "image/vnd.rn-realflash" },
{ ".rgb", "image/x-rgb" },
{ ".rlf", "application/x-richlink" },
{ ".rm", "audio/x-pn-realaudio" },
{ ".rmf", "audio/x-rmf" },
{ ".rmi", "audio/mid" },
{ ".rmm", "audio/x-pn-realaudio" },
{ ".rmvb", "audio/x-pn-realaudio" },
{ ".rnx", "application/vnd.rn-realplayer" },
{ ".roff", "application/x-troff" },
{ ".rp", "image/vnd.rn-realpix" },
{ ".rpm", "audio/x-pn-realaudio-plugin" },
{ ".rt", "text/vnd.rn-realtext" },
{ ".rte", "x-lml/x-gps" },
{ ".rtf", "application/rtf" },
{ ".rtg", "application/metastream" },
{ ".rtx", "text/richtext" },
{ ".rv", "video/vnd.rn-realvideo" },
{ ".rwc", "application/x-rogerwilco" },
{ ".s3m", "audio/x-mod" },
{ ".s3z", "audio/x-mod" },
{ ".sca", "application/x-supercard" },
{ ".scd", "application/x-msschedule" },
{ ".sct", "text/scriptlet" },
{ ".sdf", "application/e-score" },
{ ".sea", "application/x-stuffit" },
{ ".setpay", "application/set-payment-initiation" },
{ ".setreg", "application/set-registration-initiation" },
{ ".sgm", "text/x-sgml" },
{ ".sgml", "text/x-sgml" },
{ ".sh", "application/x-sh" },
{ ".shar", "application/x-shar" },
{ ".shtml", "magnus-internal/parsed-html" },
{ ".shw", "application/presentations" },
{ ".si6", "image/si6" },
{ ".si7", "image/vnd.stiwap.sis" },
{ ".si9", "image/vnd.lgtwap.sis" },
{ ".sis", "application/vnd.symbian.install" },
{ ".sit", "application/x-stuffit" },
{ ".skd", "application/x-Koan" },
{ ".skm", "application/x-Koan" },
{ ".skp", "application/x-Koan" },
{ ".skt", "application/x-Koan" },
{ ".slc", "application/x-salsa" },
{ ".smd", "audio/x-smd" },
{ ".smi", "application/smil" },
{ ".smil", "application/smil" },
{ ".smp", "application/studiom" },
{ ".smz", "audio/x-smd" },
{ ".snd", "audio/basic" },
{ ".spc", "application/x-pkcs7-certificates" },
{ ".spl", "application/futuresplash" },
{ ".spr", "application/x-sprite" },
{ ".sprite", "application/x-sprite" },
{ ".sdp", "application/sdp" },
{ ".spt", "application/x-spt" },
{ ".src", "application/x-wais-source" },
{ ".sst", "application/vnd.ms-pkicertstore" },
{ ".stk", "application/hyperstudio" },
{ ".stl", "application/vnd.ms-pkistl" },
{ ".stm", "text/html" },
{ ".svg", "image/svg+xml" },
{ ".sv4cpio", "application/x-sv4cpio" },
{ ".sv4crc", "application/x-sv4crc" },
{ ".svf", "image/vnd" },
{ ".svg", "image/svg+xml" },
{ ".svh", "image/svh" },
{ ".svr", "x-world/x-svr" },
{ ".swf", "application/x-shockwave-flash" },
{ ".swfl", "application/x-shockwave-flash" },
{ ".t", "application/x-troff" },
{ ".tad", "application/octet-stream" },
{ ".talk", "text/x-speech" },
{ ".tar", "application/x-tar" },
{ ".taz", "application/x-tar" },
{ ".tbp", "application/x-timbuktu" },
{ ".tbt", "application/x-timbuktu" },
{ ".tcl", "application/x-tcl" },
{ ".tex", "application/x-tex" },
{ ".texi", "application/x-texinfo" },
{ ".texinfo", "application/x-texinfo" },
{ ".tgz", "application/x-compressed" },
{ ".thm", "application/vnd.eri.thm" },
{ ".tif", "image/tiff" },
{ ".tiff", "image/tiff" },
{ ".tki", "application/x-tkined" },
{ ".tkined", "application/x-tkined" },
{ ".toc", "application/toc" },
{ ".toy", "image/toy" },
{ ".tr", "application/x-troff" },
{ ".trk", "x-lml/x-gps" },
{ ".trm", "application/x-msterminal" },
{ ".tsi", "audio/tsplayer" },
{ ".tsp", "application/dsptype" },
{ ".tsv", "text/tab-separated-values" },
{ ".ttf", "application/octet-stream" },
{ ".ttz", "application/t-time" },
{ ".txt", "text/plain" },
{ ".uls", "text/iuls" },
{ ".ult", "audio/x-mod" },
{ ".ustar", "application/x-ustar" },
{ ".uu", "application/x-uuencode" },
{ ".uue", "application/x-uuencode" },
{ ".vcd", "application/x-cdlink" },
{ ".vcf", "text/x-vcard" },
{ ".vdo", "video/vdo" },
{ ".vib", "audio/vib" },
{ ".viv", "video/vivo" },
{ ".vivo", "video/vivo" },
{ ".vmd", "application/vocaltec-media-desc" },
{ ".vmf", "application/vocaltec-media-file" },
{ ".vmi", "application/x-dreamcast-vms-info" },
{ ".vms", "application/x-dreamcast-vms" },
{ ".vox", "audio/voxware" },
{ ".vqe", "audio/x-twinvq-plugin" },
{ ".vqf", "audio/x-twinvq" },
{ ".vql", "audio/x-twinvq" },
{ ".vre", "x-world/x-vream" },
{ ".vrml", "x-world/x-vrml" },
{ ".vrt", "x-world/x-vrt" },
{ ".vrw", "x-world/x-vream" },
{ ".vts", "workbook/formulaone" },
{ ".wav", "audio/x-wav" },
{ ".wax", "audio/x-ms-wax" },
{ ".wbmp", "image/vnd.wap.wbmp" },
{ ".wcm", "application/vnd.ms-works" },
{ ".wdb", "application/vnd.ms-works" },
{ ".web", "application/vnd.xara" },
{ ".wi", "image/wavelet" },
{ ".wis", "application/x-InstallShield" },
{ ".wks", "application/vnd.ms-works" },
{ ".wm", "video/x-ms-wm" },
{ ".wma", "audio/x-ms-wma" },
{ ".wmd", "application/x-ms-wmd" },
{ ".wmf", "application/x-msmetafile" },
{ ".wml", "text/vnd.wap.wml" },
{ ".wmlc", "application/vnd.wap.wmlc" },
{ ".wmls", "text/vnd.wap.wmlscript" },
{ ".wmlsc", "application/vnd.wap.wmlscriptc" },
{ ".wmlscript", "text/vnd.wap.wmlscript" },
{ ".wmv", "audio/x-ms-wmv" },
{ ".wmx", "video/x-ms-wmx" },
{ ".wmz", "application/x-ms-wmz" },
{ ".wpng", "image/x-up-wpng" },
{ ".wps", "application/vnd.ms-works" },
{ ".wpt", "x-lml/x-gps" },
{ ".wri", "application/x-mswrite" },
{ ".wrl", "x-world/x-vrml" },
{ ".wrz", "x-world/x-vrml" },
{ ".ws", "text/vnd.wap.wmlscript" },
{ ".wsc", "application/vnd.wap.wmlscriptc" },
{ ".wv", "video/wavelet" },
{ ".wvx", "video/x-ms-wvx" },
{ ".wxl", "application/x-wxl" },
{ ".x-gzip", "application/x-gzip" },
{ ".xaf", "x-world/x-vrml" },
{ ".xar", "application/vnd.xara" },
{ ".xbm", "image/x-xbitmap" },
{ ".xdm", "application/x-xdma" },
{ ".xdma", "application/x-xdma" },
{ ".xdw", "application/vnd.fujixerox.docuworks" },
{ ".xht", "application/xhtml+xml" },
{ ".xhtm", "application/xhtml+xml" },
{ ".xhtml", "application/xhtml+xml" },
{ ".xla", "application/vnd.ms-excel" },
{ ".xlc", "application/vnd.ms-excel" },
{ ".xll", "application/x-excel" },
{ ".xlm", "application/vnd.ms-excel" },
{ ".xls", "application/vnd.ms-excel" },
{ ".xlsx", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" },
{ ".xlt", "application/vnd.ms-excel" },
{ ".xlw", "application/vnd.ms-excel" },
{ ".xm", "audio/x-mod" },
{".xml","text/plain"},
{".xml","application/xml"},
{ ".xmz", "audio/x-mod" },
{ ".xof", "x-world/x-vrml" },
{ ".xpi", "application/x-xpinstall" },
{ ".xpm", "image/x-xpixmap" },
{ ".xsit", "text/xml" },
{ ".xsl", "text/xml" },
{ ".xul", "text/xul" },
{ ".xwd", "image/x-xwindowdump" },
{ ".xyz", "chemical/x-pdb" },
{ ".yz1", "application/x-yz1" },
{ ".z", "application/x-compress" },
{ ".zac", "application/x-zaurus-zac" },
{ ".zip", "application/zip" },
{ ".json", "application/json" },
傳輸編碼
内容傳輸編碼（Content-Transfer-Encoding），這個區域使指定ASCII以外的字符編碼方式成為可能。形式如下：
Content-Transfer-Encoding: [mechanism]
其中，mechanism的值可以指定為“7bit”，“8bit”，“binary”，“quoted-printable”，“base64”。
7bit
7bit這裡指的是7位的ASCII編碼方式。
8bit
8位元ASCII碼。
binary
quoted-printable
因為歐洲的一些文字和ASCII字符集中的某些字符有部分相同。如果郵件消息使用的是這些語言的話，于ASCII重疊的那些字符可以原樣使用，ASCII字符集中不存在的字符采用形如“=??”的方法編碼。這裡“??”需要用将字符編碼後的16進制數字來指定。采用quoted-printable編碼的消息，長度不會變得太長，而且大部分都是ASCII中的字符，即使不通過解碼也大緻可以讀懂消息的内容。
base64
base64是一種将二進制的01序列轉化成ASCII字符的編碼方法。編碼後的文本或者二進制消息，就可以運用SMTP等隻支持ASCII字符的協議傳送了。Base64一般被認為會平均增加33%的報文長度，而且，經過編碼的消息對于人類來說是不可讀的。
x-encodingname
這個值是預留的擴展。
