Let's say you want to send to your server a modified version of the file the user is going to upload.

filereader.js

listeners: () =>
  @dropzone.on 'addedfile', (file) =>
    _reader = new FileReader

    _reader.onload = (e) =>
      _attachment = _reader.result
      @attachmentsQueue.push { file: file, content: _attachment }

    _reader.readAsBinaryString file

sendAttachment: (attachment, file) =>
  _blob = new Blob [attachment], { type: file.type }

  _formData = new FormData
  _formData.append 'message[attachments_attributes][][file]', _blob, file.name

  _progressBar = $('[data-dz-uploadprogress]')

  $.ajax
    url: "/api/v1/messages/#{@id}"
    method: 'PUT'
    cache: false
    contentType: false
    processData: false
    data: _formData
    beforeSend: () =>
      _progressBar.css 'width', '0'
    xhr: () =>
      _progress = (e) =>
        if e.lengthComputable
          _percentComplete = (e.loaded / e.total) * 100
          _progressBar.css 'width', "#{_percentComplete}%"

      _xhr = new window.XMLHttpRequest
      _xhr.upload.addEventListener 'progress', _progress, false
      _xhr.addEventListener        'progress', _progress, false

      return _xhr
  .done (data) =>
    log 'UPLOADED SUCCESS', 'success'
    @onUploadSuccess file, data

    @processAttachmentsQueue()