http://blog.csdn.net/oldcrane/article/details/3850244
原文出处不详，anyway，感谢一下作者先。
ajax开发中, 常遇到下面的几种情况:
1 服务端需要返回一段普通文本给客户端
2 服务端需要返回一段HTML代码给客户端
3 服务端需要返回一段XML代码给客户端
4 服务端需要返回一段javascript代码给客户端
5 服务端需要返回一段json串给客户端
对于每一种返回类型，规范的做法是要在服务端指定 response的contentType。
(当然 不指定绝大多数情况下也没什么问题 尤其是返回"非xml"的时候)
代码
普通文本 : text/plain
HTML代码 : text/html
XML代码 : text/xml
以上三个可以说是毫无争议的, 但是另外两种情况 就要注意一下了.
javascript 的 contentType 按最标准的写法 应该是 application/javascript.
而常用的 text/javascript 已经被 rfc定义为废弃的.
但是，在这里暂时不建议使用 application/javascript
因为很多老旧浏览器并不支持 application/javascript
而所有浏览器都支持 text/javascript.
在标准和广泛的兼容性之间 还是暂且选择后者吧.
json 的 contentType 常见写法有: text/json & text/javascript .
但是 这个 text/json 其实是根本不存在的, 而 text/javascript 在有些时候客户端处理起来会有歧义.
对于json的contentType , rfc里定义的标准写法是 :application/json.
在这里毫无疑问 我们应该选择标准写法的 application/json
最后，附一下ContentType清单：
常用的：
text/plain text/HTML text/xml test/javascript application/json image/GIF image/JPEG
清单：
".*"="application/octet-stream" ".001"="application/x-001" ".301"="application/x-301" ".323"="text/h323" ".906"="application/x-906" ".907"="drawing/907" ".a11"="application/x-a11" ".acp"="audio/x-mei-aac" ".ai"="application/postscript" ".aif"="audio/aiff" ".aifc"="audio/aiff" ".aiff"="audio/aiff" ".anv"="application/x-anv" ".asa"="text/asa" ".asf"="video/x-ms-asf" ".asp"="text/asp" ".asx"="video/x-ms-asf" ".au"="audio/basic" ".avi"="video/avi" ".awf"="application/vnd.adobe.workflow" ".biz"="text/xml" ".bmp"="application/x-bmp" ".bot"="application/x-bot" ".c4t"="application/x-c4t" ".c90"="application/x-c90" ".cal"="application/x-cals" ".cat"="application/vnd.ms-pki.seccat" ".cdf"="application/x-netcdf" ".cdr"="application/x-cdr" ".cel"="application/x-cel" ".cer"="application/x-x509-ca-cert" ".cg4"="application/x-g4" ".cgm"="application/x-cgm" ".cit"="application/x-cit" ".class"="java/*" ".cml"="text/xml" ".cmp"="application/x-cmp" ".cmx"="application/x-cmx" ".cot"="application/x-cot" ".crl"="application/pkix-crl" ".crt"="application/x-x509-ca-cert" ".csi"="application/x-csi" ".css"="text/css" ".cut"="application/x-cut" ".dbf"="application/x-dbf" ".dbm"="application/x-dbm" ".dbx"="application/x-dbx" ".dcd"="text/xml" ".dcx"="application/x-dcx" ".der"="application/x-x509-ca-cert" ".dgn"="application/x-dgn" ".dib"="application/x-dib" ".dll"="application/x-msdownload" ".doc"="application/msword" ".dot"="application/msword" ".drw"="application/x-drw" ".dtd"="text/xml" ".dwf"="Model/vnd.dwf" ".dwf"="application/x-dwf" ".dwg"="application/x-dwg" ".dxb"="application/x-dxb" ".dxf"="application/x-dxf" ".edn"="application/vnd.adobe.edn" ".emf"="application/x-emf" ".eml"="message/rfc822" ".ent"="text/xml" ".epi"="application/x-epi" ".eps"="application/x-ps" ".eps"="application/postscript" ".etd"="application/x-ebx" ".exe"="application/x-msdownload" ".fax"="image/fax" ".fdf"="application/vnd.fdf" ".fif"="application/fractals" ".fo"="text/xml" ".frm"="application/x-frm" ".g4"="application/x-g4" ".gbr"="application/x-gbr" ".gcd"="application/x-gcd" ".gif"="image/gif" ".gl2"="application/x-gl2" ".gp4"="application/x-gp4" ".hgl"="application/x-hgl" ".hmr"="application/x-hmr" ".hpg"="application/x-hpgl" ".hpl"="application/x-hpl" ".hqx"="application/mac-binhex40" ".hrf"="application/x-hrf" ".hta"="application/hta" ".htc"="text/x-component" ".htm"="text/html" ".html"="text/html" ".htt"="text/webviewhtml" ".htx"="text/html" ".icb"="application/x-icb" ".ico"="image/x-icon" ".ico"="application/x-ico" ".iff"="application/x-iff" ".ig4"="application/x-g4" ".igs"="application/x-igs" ".iii"="application/x-iphone" ".img"="application/x-img" ".ins"="application/x-internet-signup" ".isp"="application/x-internet-signup" ".IVF"="video/x-ivf" ".java"="java/*" ".jfif"="image/jpeg" ".jpe"="image/jpeg" ".jpe"="application/x-jpe" ".jpeg"="image/jpeg" ".jpg"="image/jpeg" ".jpg"="application/x-jpg" ".js"="application/x-javascript" ".jsp"="text/html" ".la1"="audio/x-liquid-file" ".lar"="application/x-laplayer-reg" ".latex"="application/x-latex" ".lavs"="audio/x-liquid-secure" ".lbm"="application/x-lbm" ".lmsff"="audio/x-la-lms" ".ls"="application/x-javascript" ".ltr"="application/x-ltr" ".m1v"="video/x-mpeg" ".m2v"="video/x-mpeg" ".m3u"="audio/mpegurl" ".m4e"="video/mpeg4" ".mac"="application/x-mac" ".man"="application/x-troff-man" ".math"="text/xml" ".mdb"="application/msaccess" ".mdb"="application/x-mdb" ".mfp"="application/x-shockwave-flash" ".mht"="message/rfc822" ".mhtml"="message/rfc822" ".mi"="application/x-mi" ".mid"="audio/mid" ".midi"="audio/mid" ".mil"="application/x-mil" ".mml"="text/xml" ".mnd"="audio/x-musicnet-download" ".mns"="audio/x-musicnet-stream" ".mocha"="application/x-javascript" ".movie"="video/x-sgi-movie" ".mp1"="audio/mp1" ".mp2"="audio/mp2" ".mp2v"="video/mpeg" ".mp3"="audio/mp3" ".mp4"="video/mpeg4" ".mpa"="video/x-mpg" ".mpd"="application/vnd.ms-project" ".mpe"="video/x-mpeg" ".mpeg"="video/mpg" ".mpg"="video/mpg" ".mpga"="audio/rn-mpeg" ".mpp"="application/vnd.ms-project" ".mps"="video/x-mpeg" ".mpt"="application/vnd.ms-project" ".mpv"="video/mpg" ".mpv2"="video/mpeg" ".mpw"="application/vnd.ms-project" ".mpx"="application/vnd.ms-project" ".mtx"="text/xml" ".mxp"="application/x-mmxp" ".net"="image/pnetvue" ".nrf"="application/x-nrf" ".nws"="message/rfc822" ".odc"="text/x-ms-odc" ".out"="application/x-out" ".p10"="application/pkcs10" ".p12"="application/x-pkcs12" ".p7b"="application/x-pkcs7-certificates" ".p7c"="application/pkcs7-mime" ".p7m"="application/pkcs7-mime" ".p7r"="application/x-pkcs7-certreqresp" ".p7s"="application/pkcs7-signature" ".pc5"="application/x-pc5" ".pci"="application/x-pci" ".pcl"="application/x-pcl" ".pcx"="application/x-pcx" ".pdf"="application/pdf" ".pdf"="application/pdf" ".pdx"="application/vnd.adobe.pdx" ".pfx"="application/x-pkcs12" ".pgl"="application/x-pgl" ".pic"="application/x-pic" ".pko"="application/vnd.ms-pki.pko" ".pl"="application/x-perl" ".plg"="text/html" ".pls"="audio/scpls" ".plt"="application/x-plt" ".png"="image/png" ".png"="application/x-png" ".pot"="application/vnd.ms-powerpoint" ".ppa"="application/vnd.ms-powerpoint" ".ppm"="application/x-ppm" ".pps"="application/vnd.ms-powerpoint" ".ppt"="application/vnd.ms-powerpoint" ".ppt"="application/x-ppt" ".pr"="application/x-pr" ".prf"="application/pics-rules" ".prn"="application/x-prn" ".prt"="application/x-prt" ".ps"="application/x-ps" ".ps"="application/postscript" ".ptn"="application/x-ptn" ".pwz"="application/vnd.ms-powerpoint" ".r3t"="text/vnd.rn-realtext3d" ".ra"="audio/vnd.rn-realaudio" ".ram"="audio/x-pn-realaudio" ".ras"="application/x-ras" ".rat"="application/rat-file" ".rdf"="text/xml" ".rec"="application/vnd.rn-recording" ".red"="application/x-red" ".rgb"="application/x-rgb" ".rjs"="application/vnd.rn-realsystem-rjs" ".rjt"="application/vnd.rn-realsystem-rjt" ".rlc"="application/x-rlc" ".rle"="application/x-rle" ".rm"="application/vnd.rn-realmedia" ".rmf"="application/vnd.adobe.rmf" ".rmi"="audio/mid" ".rmj"="application/vnd.rn-realsystem-rmj" ".rmm"="audio/x-pn-realaudio" ".rmp"="application/vnd.rn-rn_music_package" ".rms"="application/vnd.rn-realmedia-secure" ".rmvb"="application/vnd.rn-realmedia-vbr" ".rmx"="application/vnd.rn-realsystem-rmx" ".rnx"="application/vnd.rn-realplayer" ".rp"="image/vnd.rn-realpix" ".rpm"="audio/x-pn-realaudio-plugin" ".rsml"="application/vnd.rn-rsml" ".rt"="text/vnd.rn-realtext" ".rtf"="application/msword" ".rtf"="application/x-rtf" ".rv"="video/vnd.rn-realvideo" ".sam"="application/x-sam" ".sat"="application/x-sat" ".sdp"="application/sdp" ".sdw"="application/x-sdw" ".sit"="application/x-stuffit" ".slb"="application/x-slb" ".sld"="application/x-sld" ".slk"="drawing/x-slk" ".smi"="application/smil" ".smil"="application/smil" ".smk"="application/x-smk" ".snd"="audio/basic" ".sol"="text/plain" ".sor"="text/plain" ".spc"="application/x-pkcs7-certificates" ".spl"="application/futuresplash" ".spp"="text/xml" ".ssm"="application/streamingmedia" ".sst"="application/vnd.ms-pki.certstore" ".stl"="application/vnd.ms-pki.stl" ".stm"="text/html" ".sty"="application/x-sty" ".svg"="text/xml" ".swf"="application/x-shockwave-flash" ".tdf"="application/x-tdf" ".tg4"="application/x-tg4" ".tga"="application/x-tga" ".tif"="image/tiff" ".tif"="application/x-tif" ".tiff"="image/tiff" ".tld"="text/xml" ".top"="drawing/x-top" ".torrent"="application/x-bittorrent" ".tsd"="text/xml" ".txt"="text/plain" ".uin"="application/x-icq" ".uls"="text/iuls" ".vcf"="text/x-vcard" ".vda"="application/x-vda" ".vdx"="application/vnd.visio" ".vml"="text/xml" ".vpg"="application/x-vpeg005" ".vsd"="application/vnd.visio" ".vsd"="application/x-vsd" ".vss"="application/vnd.visio" ".vst"="application/vnd.visio" ".vst"="application/x-vst" ".vsw"="application/vnd.visio" ".vsx"="application/vnd.visio" ".vtx"="application/vnd.visio" ".vxml"="text/xml" ".wav"="audio/wav" ".wax"="audio/x-ms-wax" ".wb1"="application/x-wb1" ".wb2"="application/x-wb2" ".wb3"="application/x-wb3" ".wbmp"="image/vnd.wap.wbmp" ".wiz"="application/msword" ".wk3"="application/x-wk3" ".wk4"="application/x-wk4" ".wkq"="application/x-wkq" ".wks"="application/x-wks" ".wm"="video/x-ms-wm" ".wma"="audio/x-ms-wma" ".wmd"="application/x-ms-wmd" ".wmf"="application/x-wmf" ".wml"="text/vnd.wap.wml" ".wmv"="video/x-ms-wmv" ".wmx"="video/x-ms-wmx" ".wmz"="application/x-ms-wmz" ".wp6"="application/x-wp6" ".wpd"="application/x-wpd" ".wpg"="application/x-wpg" ".wpl"="application/vnd.ms-wpl" ".wq1"="application/x-wq1" ".wr1"="application/x-wr1" ".wri"="application/x-wri" ".wrk"="application/x-wrk" ".ws"="application/x-ws" ".ws2"="application/x-ws" ".wsc"="text/scriptlet" ".wsdl"="text/xml" ".wvx"="video/x-ms-wvx" ".xdp"="application/vnd.adobe.xdp" ".xdr"="text/xml" ".xfd"="application/vnd.adobe.xfd" ".xfdf"="application/vnd.adobe.xfdf" ".xhtml"="text/html" ".xls"="application/vnd.ms-excel" ".xls"="application/x-xls" ".xlw"="application/x-xlw" ".xml"="text/xml" ".xpl"="audio/scpls" ".xq"="text/xml" ".xql"="text/xml" ".xquery"="text/xml" ".xsd"="text/xml" ".xsl"="text/xml" ".xslt"="text/xml" ".xwd"="application/x-xwd" ".x_b"="application/x-x_b" ".x_t"="application/x-x_t"