mergeFileList (input) {
    this.addFileList(input.files)
    // Create a new mutable FileList
    if (this.supportsDataTransfers) {
      const transfer = new DataTransfer()
      this.files.forEach(file => {
        if (file.file instanceof File) {
          transfer.items.add(file.file)
        }
      })
      this.fileList = transfer.files
      this.input.files = this.fileList
      // Reset the merged FileList to empty
      input.files = (new DataTransfer()).files
    }
    this.context.performValidation()
    this.loadPreviews()
    if (this.context.uploadBehavior !== 'delayed') {
      this.upload()
    }
  }
Given another input element, add the files from that FileList to the
input being represented by this FileUpload.

@param {HTMLElement} input