plupload 获取视频时长

file 已经被拦截处理，

file.getNative()获取真正的对象

let fileurl = URL.createObjectURL(file.getNative())
let audioElement = new Audio(fileurl)
audioElement.addEventListener('loadedmetadata', _event => {
  that.$emit('getValue', {
    url: key,
    originalFileName: fileN.name,
    duration:audioElement.duration
  })
})