（IE9及以下不支持下面这些功能，其它浏览器最新版本均已支持。）
1、允许上传文件数量
（IE9及以下不支持下面这些功能，其它浏览器最新版本均已支持。）
1、允许上传文件数量
允许选择多个文件：
<input type="file" multiple>
只允许上传一个文件：
<input type="file" single>
2、上传指定的文件格式
<input type="file" accept="image/gif,image/png" />
这里的上传格式仅指打开上传弹窗选择文件时默认的文件格式，可手动修改成其它文件格式。
如果不限制上传图片的格式可写成：accept="image/*"
其它格式参考如下：
*.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
示例
<p>显示 .xls, .xlsx, .csv 文件...</p>
<input type="file" accept=".csv, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet, application/vnd.ms-excel" ID="fileSelect" runat="server" />
<p>只显示 Excel (.xlsx) 文件...</p>
<input type="file" accept="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" ID="fileSelect" runat="server" />
<p>只显示 Excel (.xls) 文件...</p>
<input type="file" accept="application/vnd.ms-excel" ID="fileSelect" runat="server" />
<p>只显示图片.</p>
<input type="file" accept="image/*" ID="fileSelect" runat="server" />
<p>只显示文本文件...</p>
<input type="file" accept="text/plain" ID="fileSelect" runat="server" />
<p>只显示html文件.</p>
<input type="file" accept="text/html" ID="fileSelect" runat="server" />
<p>只显示 video 文件..</p>
<input type="file" accept="video/*" ID="fileSelect" runat="server" />
<p>只显示 audio 文件...</p>
<input type="file" accept="audio/*" ID="fileSelect" runat="server" />
<p>只显示 .WAV 文件...</p>
<input type="file" accept=".wav" ID="fileSelect" runat="server" />
<p>只显示 .PDF 文件...</p>
<input type="file" accept=".pdf" ID="fileSelect" runat="server" />