要学习content-type,必须事先知道它到底是什么，是干什么用的。

HTTP协议（RFC2616）采用了请求/响应模型。客户端向服务器发送一个请求，请求头包含请求的方法、URI、协议版本、以及包含请求修饰符、客户 信息和内容的类似于MIME的消息结构。服务器以一个状态行作为响应，相应的内容包括消息协议的版本，成功或者错误编码加上包含服务器信息、实体元信息以 及可能的实体内容。

通常HTTP消息由一个起始行，一个或者多个头域，一个只是头域结束的空行和可选的消息体组成。HTTP的头域包括通用头，请求头，响应头和实体头四个部分。每个头域由一个域名，冒号（:）和域值三部分组成。域名是大小写无关的，域 值前可以添加任何数量的空格符，头域可以被扩展为多行，在每行开始处，使用至少一个空格或制表符。

请求消息和响应消息都可以包含实体信息，实体信息一般由实体头域和实体组成。实体头域包含关于实体的原信息，实体头包括Allow、Content- Base、Content-Encoding、Content-Language、 Content-Length、Content-Location、Content-MD5、Content-Range、Content-Type、 Etag、Expires、Last-Modified、extension-header。
Content-Type是返回消息中非常重要的内容，表示后面的文档属于什么MIME类型。Content-Type: [type]/[subtype]; parameter。例如最常见的就是text/html，它的意思是说返回的内容是文本类型，这个文本又是HTML格式的。原则上浏览器会根据Content-Type来决定如何显示返回的消息体内容。

type有下面的形式

Text：用于标准化地表示的文本信息，文本消息可以是多种字符集和或者多种格式的；

Multipart：用于连接消息体的多个部分构成一个消息，这些部分可以是不同类型的数据；

Application：用于传输应用程序数据或者二进制数据；

Message：用于包装一个E-mail消息；

Image：用于传输静态图片数据；

Audio：用于传输音频或者音声数据；

Video：用于传输动态影像数据，可以是与音频编辑在一起的视频数据格式。

subtype用于指定type的详细形式。content-type/subtype配对的集合和与此相关的参数，将随着时间而增长。为了确保这些值在一个有序而且公开的状态下开发，MIME使用Internet Assigned Numbers Authority (IANA)作为中心的注册机制来管理这些值。

parameter可以用来指定附加的信息，更多情况下是用于指定text/plain和text/htm等的文字编码方式的charset参数。MIME根据type制定了默认的subtype，当客户端不能确定消息的subtype的情况下，消息被看作默认的subtype进行处理。Text默认是text/plain，Application默认是application/octet-stream而Multipart默认情况下被看作multipart/mixed。对于IE6浏览器来说，如果Content-Type中的类型和实际的消息体类型不一致，那么它会根据内容中的类型来分析实际应该是什么类型，对于JPG、GIF等常用图片格式都可以正确的识别出来，而不管Content-Type中写的是什么。
如果Content-Type中指定的是浏览器可以直接打开的类型，那么浏览器就会直接打开其内容显示出来，如果是被关联到其它应用程序的类型，这时就要查找注册表中关于这种类型的注册情况，如果是允许直接打开而不需要询问的，就会直接调出这个关联的应用程序来打开这个文件，但如果是不允许直接打开的，就会询问是否打开。对于没有关联到任何应用程序的类型，IE浏览器不知道它该如何打开，此时IE6就会把它当成XML来尝试打开。

如果要想实现文件下载，可以在http的头中加入：

fprintf(file, "Content-Disposition:attachment; filename="%s" ", fileName);

经过测试，html,pdf,gif等原来在网页中打开的文件都可以正常实现下载。

"hqx" => "application/mac-binhex40",
"cpt" => "application/mac-compactpro",
"doc" => "application/msword",
"bin" => "application/octet-stream",
"dms" => "application/octet-stream",
"lha" => "application/octet-stream",
"lzh" => "application/octet-stream",
"exe" => "application/octet-stream",
"class" => "application/octet-stream",
"so" => "application/octet-stream",
"dll" => "application/octet-stream",
"oda" => "application/oda",
"pdf" => "application/pdf",
"ai" => "application/postscript",
"eps" => "application/postscript",
"ps" => "application/postscript",
"smi" => "application/smil",
"smil" => "application/smil",
"mif" => "application/vnd.mif",
"xls" => "application/vnd.ms-excel",
"ppt" => "application/vnd.ms-powerpoint",
"wbxml" => "application/vnd.wap.wbxml",
"wmlc" => "application/vnd.wap.wmlc",
"wmlsc" => "application/vnd.wap.wmlscriptc",
"bcpio" => "application/x-bcpio",
"vcd" => "application/x-cdlink",
"pgn" => "application/x-chess-pgn",
"cpio" => "application/x-cpio",
"csh" => "application/x-csh",
"dcr" => "application/x-director",
"dir" => "application/x-director",
"dxr" => "application/x-director",
"dvi" => "application/x-dvi",
"spl" => "application/x-futuresplash",
"gtar" => "application/x-gtar",
"hdf" => "application/x-hdf",
"js" => "application/x-javascript",
"skp" => "application/x-koan",
"skd" => "application/x-koan",
"skt" => "application/x-koan",
"skm" => "application/x-koan",
"latex" => "application/x-latex",
"nc" => "application/x-netcdf",
"cdf" => "application/x-netcdf",
"sh" => "application/x-sh",
"shar" => "application/x-shar",
"swf" => "application/x-shockwave-flash",
"sit" => "application/x-stuffit",
"sv4cpio" => "application/x-sv4cpio",
"sv4crc" => "application/x-sv4crc",
"tar" => "application/x-tar",
"tcl" => "application/x-tcl",
"tex" => "application/x-tex",
"texinfo" => "application/x-texinfo",
"texi" => "application/x-texinfo",
"t" => "application/x-troff",
"tr" => "application/x-troff",
"roff" => "application/x-troff",
"man" => "application/x-troff-man",
"me" => "application/x-troff-me",
"ms" => "application/x-troff-ms",
"ustar" => "application/x-ustar",
"src" => "application/x-wais-source",
"xhtml" => "application/xhtml+xml",
"xht" => "application/xhtml+xml",
"zip" => "application/zip",
"au" => "audio/basic",
"snd" => "audio/basic",
"mid" => "audio/midi",
"midi" => "audio/midi",
"kar" => "audio/midi",
"mpga" => "audio/mpeg",
"mp2" => "audio/mpeg",
"mp3" => "audio/mpeg",
"aif" => "audio/x-aiff",
"aiff" => "audio/x-aiff",
"aifc" => "audio/x-aiff",
"m3u" => "audio/x-mpegurl",
"ram" => "audio/x-pn-realaudio",
"rm" => "audio/x-pn-realaudio",
"rpm" => "audio/x-pn-realaudio-plugin",
"ra" => "audio/x-realaudio",
"wav" => "audio/x-wav",
"pdb" => "chemical/x-pdb",
"xyz" => "chemical/x-xyz",
"bmp" => "image/bmp",
"gif" => "image/gif",
"ief" => "image/ief",
"jpeg" => "image/jpeg",
"jpg" => "image/jpeg",
"jpe" => "image/jpeg",
"png" => "image/png",
"tiff" => "image/tiff",
"tif" => "image/tiff",
"djvu" => "image/vnd.djvu",
"djv" => "image/vnd.djvu",
"wbmp" => "image/vnd.wap.wbmp",
"ras" => "image/x-cmu-raster",
"pnm" => "image/x-portable-anymap",
"pbm" => "image/x-portable-bitmap",
"pgm" => "image/x-portable-graymap",
"ppm" => "image/x-portable-pixmap",
"rgb" => "image/x-rgb",
"xbm" => "image/x-xbitmap",
"xpm" => "image/x-xpixmap",
"xwd" => "image/x-xwindowdump",
"igs" => "model/iges",
"iges" => "model/iges",
"msh" => "model/mesh",
"mesh" => "model/mesh",
"silo" => "model/mesh",
"wrl" => "model/vrml",
"vrml" => "model/vrml",
"css" => "text/css",
"html" => "text/html",
"htm" => "text/html",
"asc" => "text/plain",
"txt" => "text/plain",
"rtx" => "text/richtext",
"rtf" => "text/rtf",
"sgml" => "text/sgml",
"sgm" => "text/sgml",
"tsv" => "text/tab-separated-values",
"wml" => "text/vnd.wap.wml",
"wmls" => "text/vnd.wap.wmlscript",
"etx" => "text/x-setext",
"xsl" => "text/xml",
"xml" => "text/xml",
"mpeg" => "video/mpeg",
"mpg" => "video/mpeg",
"mpe" => "video/mpeg",
"qt" => "video/quicktime",
"mov" => "video/quicktime",
"mxu" => "video/vnd.mpegurl",
"avi" => "video/x-msvideo",
"movie" => "video/x-sgi-movie",
"ice" => "x-conference/x-cooltalk",