input：file上传⽂件类型（超详细）input:file上传⽂件类型（超详细）
imput 属性有以下⼏种：
2.accept：表⽰可以选择的⽂件类型，多个类型⽤英⽂逗号分开，常⽤的类型见下表。
<input id="file" type="file" accept="image/png,image/gif" name="file" />
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3.multiple：是否可以选择多个⽂件，多个⽂件时其value值为第⼀个⽂件的虚拟路径。
<input id="files" type="file" multiple="multiple" name="file"
下⾯就是各种⽂件类型（上传给接⼝时）
*.3gpp    audio/3gpp, video/3gpp
*.ac3    audio/ac3
*.asf      allpication/vnd.ms-asf
*.au          audio/basic
*.css          text/css
*.csv          text/csv
*.doc    application/msword
*.dot    application/msword
*.dtd    application/xml-dtd
*.dwg    image/vnd.dwg
*.dxf      image/vnd.dxf
*.gif            image/gif
*.htm    text/html
*.html    text/html
*.jp2            image/jp2
*.jpe      image/jpeg
*.jpeg    image/jpeg
*.jpg          image/jpeg
*.js      text/javascript, application/javascript
*.json    application/json
*.mp2    audio/mpeg, video/mpeg
*.mp3    audio/mpeghtml input type属性
*.mp4    audio/mp4, video/mp4
*.mpeg    video/mpeg
*.mpg    video/mpeg
*.mpp    application/vnd.ms-project
*.ogg    application/ogg, audio/ogg
*.pdf    application/pdf
*.png    image/png
*.pot    application/vnd.ms-powerpoint
*.pps    application/vnd.ms-powerpoint
*.ppt    application/vnd.ms-powerpoint
*.rtf            application/rtf, text/rtf
*.svf          image/vnd.svf
*.tif        image/tiff
*.tiff      image/tiff
*.txt          text/plain
*.wdb    application/vnd.ms-works
*.wps    application/vnd.ms-works
*.xhtml    application/xhtml+xml
*.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
*.xml    text/xml, application/xml
*.zip            application/zip
*.xlsx    application/vnd.openxmlformats-officedocument.spreadsheetml.sheet