http中的Content-Type
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
|
| pdf
|
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.htm
原文：http://blog.csdn.net/bluefish625/article/details/6659288