HTML5/Javascript multiple file upload

I'm working on a project that uses drag-drop file handling to attach files to a form.

The form is pretty simple:

<form accept-charset="UTF-8" action="..." enctype="multipart/form-data" method="post">
  <input multiple="multiple" name="files[]" type="file" />
  <div id="download-drop"></div>
</form>

• I can select multiple files in the FilBrowser window and they will all post to the server
• I can't drag multiple files into the dropzone - they aren't posted to the form

When I look at the server logs, I can see the files array is posted, but the files I drop onto the dropzone are not included.

Here is my javascript (actually coffeescript):

class Uploader
  constructor: (drop_zone, upload_button) ->
    @attachments = []

    @$upload_button = $(upload_button)
    @$upload_button.bind 'change', @manual

    @$drop_zone = $(drop_zone)
    @$drop_zone.bind 'dragenter', @noop
    @$drop_zone.bind 'dragexit', @noop
    @$drop_zone.bind 'dragover', @noop
    @$drop_zone.bind 'drop', @drop

    @render()

  # Perform a noop by preventing the default behavior and stopping
  # event propagation.
  noop: (e) ->
    e.stopPropagation()
    e.preventDefault()

  # Handler for when a file is dropped.
  drop: (e) =>
    @noop e

    for file in e.dataTransfer.files
      @attachments.push file

    @render()

  # Handle a manual file upload (using the HTML element)
  manual: (e) =>
    @noop e

    for file in e.currentTarget.files
      @attachments.push file

    @render()

  # Render helper function that redraws the HTML element
  render: =>
    @$drop_zone.html ''

    html = []
    if @attachments?.length > 0
      for attachment, i in @attachments
        html.push attachment.name

    @$drop_zone.html html.join('')

@Uploader = Uploader

A few things I've already looked into:

1. The class is created and both selectors are found and exist
2. noop works as expected
3. Scopes are all correct
4. Drop works as expected

Can someone shed light on this? I think I need to add each attachment to the files array, but I don't know how.