前端上传文件，筛选文件格式

input上传文件时，限制可选择的文件类型

例如上传仅图片：

<input type="file" accept="image/*" />

accept表示可以上传的文件类型，image表示图片，*表示所有支持的格式

下面列举一些accept的格式选择
*.3gpp audio/3gpp, video/3gpp3GPP Audio/Video
*.ac3 audio/ac3AC3 Audio
*.asf allpication/vnd.ms-asfAdvanced Streaming Format
*.au audio/basicAU Audio
*.css text/cssCascading Style Sheets
*.csv text/csvComma Separated Values
*.doc application/mswordMS Word Document
*.dot application/mswordMS Word Template
*.dtd application/xml-dtdDocument Type Definition
*.dwg image/vnd.dwgAutoCAD Drawing Database
*.dxf image/vnd.dxfAutoCAD Drawing Interchange Format
*.gif image/gifGraphic Interchange Format
*.htm text/htmlHyperText Markup Language
*.html text/htmlHyperText Markup Language
*.jp2 image/jp2JPEG-2000
*.jpe image/jpegJPEG
*.jpeg image/jpegJPEG
*.jpg image/jpegJPEG
*.js text/javascript, application/javascriptJavaScript
*.json application/jsonJavaScript Object Notation
*.mp2 audio/mpeg, video/mpegMPEG Audio/Video Stream, Layer II
*.mp3 audio/mpegMPEG Audio Stream, Layer III
*.mp4 audio/mp4, video/mp4MPEG-4 Audio/Video
*.mpeg video/mpegMPEG Video Stream, Layer II
*.mpg video/mpegMPEG Video Stream, Layer II
*.mpp application/vnd.ms-projectMS Project Project
*.ogg application/ogg, audio/oggOgg Vorbis
*.pdf application/pdfPortable Document Format
*.png image/pngPortable Network Graphics
*.pot application/vnd.ms-powerpointMS PowerPoint Template
*.pps application/vnd.ms-powerpointMS PowerPoint Slideshow
*.ppt application/vnd.ms-powerpointMS PowerPoint Presentation
*.rtf application/rtf, text/rtfRich Text Format
*.svf image/vnd.svfSimple Vector Format
*.tif image/tiffTagged Image Format File
*.tiff image/tiffTagged Image Format File
*.txt text/plainPlain Text
*.wdb application/vnd.ms-worksMS Works Database
*.wps application/vnd.ms-worksWorks Text Document
*.xhtml application/xhtml+xmlExtensible HyperText Markup Language
*.xlc application/vnd.ms-excelMS Excel Chart
*.xlm application/vnd.ms-excelMS Excel Macro
*.xls application/vnd.ms-excelMS Excel Spreadsheet
*.xlt application/vnd.ms-excelMS Excel Template
*.xlw application/vnd.ms-excelMS Excel Workspace
*.xml text/xml, application/xmlExtensible Markup Language
*.zip aplication/zipCompressed Archive