网页上添加一个input file HTML控件：
<input id="File1" type="file" />
默认是这样的，所有文件类型都会显示出来，如果想限制它只显示我们设定的文件类型呢，比如“word“,”excel“,”pdf“文件
解决办法是可以给它添加一个accept属性，比如：
<input id="File1" type="file" accept=".xls,.doc,.txt,.pdf" />
这样选择的时候默认会显示为这样：
文件选择框内只显示出你自定义文件类型的文件，也还比较方便。
But,这只是最简单的掩人耳目的做法，还是能选择其它文件类型：
所以，如果要做到真正意义上限制类型做法（其实这种算不上限制，只是把你要的文件类型默认显示出来而已，并不是说不能选择其它的），还是要通过js或者后台来控制。
附支持的文件类型：
*.3gpp audio/3gpp， video/3gpp 3GPP Audio/Video *.ac3 audio/ac3 AC3 Audio *.asf allpication/vnd.ms-asf Advanced Streaming Format *.au audio/basic AU Audio *.css text/css Cascading Style Sheets *.csv text/csv Comma Separated Values *.doc application/msword MS Word Document *.dot application/msword MS Word Template *.dtd application/xml-dtd Document Type Definition *.dwg image/vnd.dwg AutoCAD Drawing Database *.dxf image/vnd.dxf AutoCAD Drawing Interchange Format *.gif image/gif Graphic Interchange Format *.htm text/html HyperText Markup Language *.html text/html HyperText Markup Language *.jp2 image/jp2 JPEG-2000 *.jpe image/jpeg JPEG *.jpeg image/jpeg JPEG *.jpg image/jpeg JPEG *.js text/javascript， application/javascript JavaScript *.json application/json JavaScript Object Notation *.mp2 audio/mpeg， video/mpeg MPEG Audio/Video Stream， Layer II *.mp3 audio/mpeg MPEG Audio Stream， Layer III *.mp4 audio/mp4， video/mp4 MPEG-4 Audio/Video *.mpeg video/mpeg MPEG Video Stream， Layer II *.mpg video/mpeg MPEG Video Stream， Layer II *.mpp application/vnd.ms-project MS Project Project *.ogg application/ogg， audio/ogg Ogg Vorbis *.pdf application/pdf Portable Document Format *.png image/png Portable Network Graphics *.pot application/vnd.ms-powerpoint MS PowerPoint Template *.pps application/vnd.ms-powerpoint MS PowerPoint Slideshow *.ppt application/vnd.ms-powerpoint MS PowerPoint Presentation *.rtf application/rtf， text/rtf Rich Text Format *.svf image/vnd.svf Simple Vector Format *.tif image/tiff Tagged Image Format File *.tiff image/tiff Tagged Image Format File *.txt text/plain Plain Text *.wdb application/vnd.ms-works MS Works Database *.wps application/vnd.ms-works Works Text Document *.xhtml application/xhtml+xml Extensible HyperText Markup Language *.xlc application/vnd.ms-excel MS Excel Chart *.xlm application/vnd.ms-excel MS Excel Macro *.xls application/vnd.ms-excel MS Excel Spreadsheet *.xlt application/vnd.ms-excel MS Excel Template *.xlw application/vnd.ms-excel MS Excel Workspace *.xml text/xml， application/xml Extensible Markup Language *.zip aplication/zip Compressed Archive