【图片上传】

<input type="file" name="picture"> 没有上传文件时输出 req.files.picture

{ fieldName: 'picture',

  originalFilename: '',

  path: 'upload/5157-mukcpk',

  headers: 

   { 'content-disposition': 'form-data; name="picture"; filename=""',

     'content-type': 'application/octet-stream' },

  ws: 

   { _writableState: 

      { highWaterMark: 16384,

        objectMode: false,

        needDrain: false,

        ending: true,

        ended: true,

        finished: true,

        decodeStrings: true,

        defaultEncoding: 'utf8',

        length: 0,

        writing: false,

        sync: true,

        bufferProcessing: false,

        onwrite: [Function],

        writecb: null,

        writelen: 0,

        buffer: [],

        errorEmitted: false },

     writable: true,

     domain: null,

     _events: { error: [Object], close: [Object] },

     _maxListeners: 10,

     path: 'upload/5157-mukcpk',

     fd: null,

     flags: 'w',

     mode: 438,

     start: undefined,

     pos: undefined,

     bytesWritten: 0,

     open: [Function],

     _write: [Function],

     destroy: [Function],

     close: [Function],

     destroySoon: [Function],

     pipe: [Function],

     write: [Function],

     end: [Function],

     setMaxListeners: [Function],

     emit: [Function],

     addListener: [Function],

     on: [Function],

     once: [Function],

     removeListener: [Function],

     removeAllListeners: [Function],

     listeners: [Function] },

  size: 0,

  name: '',

  type: 'application/octet-stream' }