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。
MIME定义在RFC-2046 MIME Part 2: Media Types 。
常用类型：
Mime Types By File Extension
| Extension |
Type/sub-type |
|
|
application/octet-stream |
| 323 |
text/h323 |
| acx |
application/internet-property-stream |
| ai |
application/postscript |
| aif |
audio/x-aiff |
| aifc |
audio/x-aiff |
| aiff |
audio/x-aiff |
| asf |
video/x-ms-asf |
| asr |
video/x-ms-asf |
| asx |
video/x-ms-asf |
| au |
audio/basic |
| avi |
video/x-msvideo |
| axs |
application/olescript |
| bas |
text/plain |
| bcpio |
application/x-bcpio |
| bin |
application/octet-stream |
| bmp |
image/bmp |
| c |
text/plain |
| cat |
application/vnd.ms-pkiseccat |
| cdf |
application/x-cdf |
| cer |
application/x-x509-ca-cert |
| class |
application/octet-stream |
| clp |
application/x-msclip |
| cmx |
image/x-cmx |
| cod |
image/cis-cod |
| cpio |
application/x-cpio |
| crd |
application/x-mscardfile |
| crl |
application/pkix-crl |
| crt |
application/x-x509-ca-cert |
| csh |
application/x-csh |
| css |
text/css |
| dcr |
application/x-director |
| der |
application/x-x509-ca-cert |
| dir |
application/x-director |
| dll |
application/x-msdownload |
| dms |
application/octet-stream |
| doc |
application/msword |
| dot |
application/msword |
| dvi |
application/x-dvi |
| dxr |
application/x-director |
| eps |
application/postscript |
| etx |
text/x-setext |
| evy |
application/envoy |
| exe |
application/octet-stream |
| fif |
application/fractals |
| flr |
x-world/x-vrml |
| gif |
image/gif |
| gtar |
application/x-gtar |
| gz |
application/x-gzip |
| h |
text/plain |
| hdf |
application/x-hdf |
| hlp |
application/winhlp |
| hqx |
application/mac-binhex40 |
| hta |
application/hta |
| htc |
text/x-component |
| htm |
text/html |
| html |
text/html |
| htt |
text/webviewhtml |
| ico |
image/x-icon |
| ief |
image/ief |
| iii |
application/x-iphone |
| ins |
application/x-internet-signup |
| isp |
application/x-internet-signup |
| jfif |
image/pipeg |
| jpe |
image/jpeg |
| jpeg |
image/jpeg |
| jpg |
image/jpeg |
| js |
application/x-javascript |
| latex |
application/x-latex |
| lha |
application/octet-stream |
| lsf |
video/x-la-asf |
| lsx |
video/x-la-asf |
| lzh |
application/octet-stream |
| m13 |
application/x-msmediaview |
| m14 |
application/x-msmediaview |
| m3u |
audio/x-mpegurl |
| man |
application/x-troff-man |
| mdb |
application/x-msaccess |
| me |
application/x-troff-me |
| mht |
message/rfc822 |
| mhtml |
message/rfc822 |
| mid |
audio/mid |
| mny |
application/x-msmoney |
| mov |
video/quicktime |
| movie |
video/x-sgi-movie |
| mp2 |
video/mpeg |
| mp3 |
audio/mpeg |
| mpa |
video/mpeg |
| mpe |
video/mpeg |
| mpeg |
video/mpeg |
| mpg |
video/mpeg |
| mpp |
application/vnd.ms-project |
| mpv2 |
video/mpeg |
| ms |
application/x-troff-ms |
| mvb |
application/x-msmediaview |
| nws |
message/rfc822 |
| oda |
application/oda |
| 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 |
| pbm |
image/x-portable-bitmap |
| |
application/pdf |
| pfx |
application/x-pkcs12 |
| pgm |
image/x-portable-graymap |
| pko |
application/ynd.ms-pkipko |
| pma |
application/x-perfmon |
| pmc |
application/x-perfmon |
| pml |
application/x-perfmon |
| pmr |
application/x-perfmon |
| pmw |
application/x-perfmon |
| pnm |
image/x-portable-anymap |
| pot, |
application/vnd.ms-powerpoint |
| ppm |
image/x-portable-pixmap |
| pps |
application/vnd.ms-powerpoint |
| ppt |
application/vnd.ms-powerpoint |
| prf |
application/pics-rules |
| ps |
application/postscript |
| pub |
application/x-mspublisher |
| qt |
video/quicktime |
| ra |
audio/x-pn-realaudio |
| ram |
audio/x-pn-realaudio |
| ras |
image/x-cmu-raster |
| rgb |
image/x-rgb |
| rmi |
audio/mid |
| roff |
application/x-troff |
| rtf |
application/rtf |
| rtx |
text/richtext |
| scd |
application/x-msschedule |
| sct |
text/scriptlet |
| setpay |
application/set-payment-initiation |
| setreg |
application/set-registration-initiation |
| sh |
application/x-sh |
| shar |
application/x-shar |
| sit |
application/x-stuffit |
| snd |
audio/basic |
| spc |
application/x-pkcs7-certificates |
| spl |
application/futuresplash |
| src |
application/x-wais-source |
| sst |
application/vnd.ms-pkicertstore |
| stl |
application/vnd.ms-pkistl |
| stm |
text/html |
| svg |
image/svg+xml |
| sv4cpio |
application/x-sv4cpio |
| sv4crc |
application/x-sv4crc |
| swf |
application/x-shockwave-flash |
| t |
application/x-troff |
| tar |
application/x-tar |
| tcl |
application/x-tcl |
| tex |
application/x-tex |
| texi |
application/x-texinfo |
| texinfo |
application/x-texinfo |
| tgz |
application/x-compressed |
| tif |
image/tiff |
| tiff |
image/tiff |
| tr |
application/x-troff |
| trm |
application/x-msterminal |
| tsv |
text/tab-separated-values |
| txt |
text/plain |
| uls |
text/iuls |
| ustar |
application/x-ustar |
| vcf |
text/x-vcard |
| vrml |
x-world/x-vrml |
| wav |
audio/x-wav |
| wcm |
application/vnd.ms-works |
| wdb |
application/vnd.ms-works |
| wks |
application/vnd.ms-works |
| wmf |
application/x-msmetafile |
| wps |
application/vnd.ms-works |
| wri |
application/x-mswrite |
| wrl |
x-world/x-vrml |
| wrz |
x-world/x-vrml |
| xaf |
x-world/x-vrml |
| xbm |
image/x-xbitmap |
| xla |
application/vnd.ms-excel |
| xlc |
application/vnd.ms-excel |
| xlm |
application/vnd.ms-excel |
| xls |
application/vnd.ms-excel |
| xlt |
application/vnd.ms-excel |
| xlw |
application/vnd.ms-excel |
| xof |
x-world/x-vrml |
| xpm |
image/x-xpixmap |
| xwd |
image/x-xwindowdump |
| z |
application/x-compress |
| zip |
application/zip |
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\" \r\n", 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。
reference：
1.http://tech.ccidnet.com/art/3539/20070727/1157895_1.html
2.http://www.cnblogs.com/jacktu/archive/2008/01/16/1041710.html
