<?php
header('Content-type: application/image/pjpeg');//输出的类型
header('Content-Disposition: attachment; filename="downloaded.jpg"'); //下载显示的名字,注意格式
readfile('my.jpg');
// 并将这个文件以前面header发送信息设定的类型输出,从而会弹出一个下载框
// 就是把服务器上的my.jpg下载下来,下载显示和保存的名字默认是downloaded.jpg
?>

附带文档类型及服务器响应类别
一共两个版本.
文档类型   和 服务器响应HTTP的类型
上面的这个可以在上传文件的类型中对应上.下面的差了点.(原文下面的是用来写在文件头上的.来确定该文件是什么类型.)

'chm'=>'application/octet-stream',
    'ppt'=>'application/vnd.ms-powerpoint',
    'xls'=>'application/vnd.ms-excel',
    'doc'=>'application/msword',
    'exe'=>'application/octet-stream',
    'rar'=>'application/octet-stream',
    'js'=>"javascrīpt/js",
    'css'=>"text/css",
    'hqx'=>"application/mac-binhex40",
    'bin'=>"application/octet-stream",
    'oda'=>"application/oda",
    'pdf'=>"application/pdf",
    'ai'=>"application/postsrcipt",
    'eps'=>"application/postsrcipt",
    'es'=>"application/postsrcipt",
    'rtf'=>"application/rtf",
    'mif'=>"application/x-mif",
    'csh'=>"application/x-csh",
    'dvi'=>"application/x-dvi",
    'hdf'=>"application/x-hdf",
    'nc'=>"application/x-netcdf",
    'cdf'=>"application/x-netcdf",
    'latex'=>"application/x-latex",
    'ts'=>"application/x-troll-ts",
    'src'=>"application/x-wais-source",
    'zip'=>"application/zip",
    'bcpio'=>"application/x-bcpio",
    'cpio'=>"application/x-cpio",
    'gtar'=>"application/x-gtar",
    'shar'=>"application/x-shar",
    'sv4cpio'=>"application/x-sv4cpio",
    'sv4crc'=>"application/x-sv4crc",
    'tar'=>"application/x-tar",
    'ustar'=>"application/x-ustar",
    'man'=>"application/x-troff-man",
    'sh'=>"application/x-sh",
    'tcl'=>"application/x-tcl",
    'tex'=>"application/x-tex",
    'texi'=>"application/x-texinfo",
    'texinfo'=>"application/x-texinfo",
    't'=>"application/x-troff",
    'tr'=>"application/x-troff",
    'roff'=>"application/x-troff",
    'shar'=>"application/x-shar",
    'me'=>"application/x-troll-me",
    'ts'=>"application/x-troll-ts",
    'gif'=>"image/gif",
    'jpeg'=>"image/pjpeg",
    'jpg'=>"image/pjpeg",
    'jpe'=>"image/pjpeg",
    'ras'=>"image/x-cmu-raster",
    'pbm'=>"image/x-portable-bitmap",
    'ppm'=>"image/x-portable-pixmap",
    'xbm'=>"image/x-xbitmap",
    'xwd'=>"image/x-xwindowdump",
    'ief'=>"image/ief",
    'tif'=>"image/tiff",
    'tiff'=>"image/tiff",
    'pnm'=>"image/x-portable-anymap",
    'pgm'=>"image/x-portable-graymap",
    'rgb'=>"image/x-rgb",
    'xpm'=>"image/x-xpixmap",
    'txt'=>"text/plain",
    'c'=>"text/plain",
    'cc'=>"text/plain",
    'h'=>"text/plain",
    'html'=>"text/html",
    'htm'=>"text/html",
    'htl'=>"text/html",
    'rtx'=>"text/richtext",
    'etx'=>"text/x-setext",
    'tsv'=>"text/tab-separated-values",
    'mpeg'=>"video/mpeg",
    'mpg'=>"video/mpeg",
    'mpe'=>"video/mpeg",
    'avi'=>"video/x-msvideo",
    'qt'=>"video/quicktime",
    'mov'=>"video/quicktime",
    'moov'=>"video/quicktime",
    'movie'=>"video/x-sgi-movie",
    'au'=>"audio/basic",
    'snd'=>"audio/basic",
    'wav'=>"audio/x-wav",
    'aif'=>"audio/x-aiff",
    'aiff'=>"audio/x-aiff",
    'aifc'=>"audio/x-aiff",
    'swf'=>"application/x-shockwave-flash"
    'myz'=>"application/myz"


".*"="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"