Add form through block for easy of configuration.

If we just add the form through a block that would help site builders decide where this blob_relay may be active. This way the only code needed at some point is

 javascript
Drupal.blobRelay.FillForm({
  mimetype: 'text/test',
  data: 'Text file test content',
  encoding: '',
  filename: 'text.txt'
});

Note this depends on #2190119: No need for delaying form submit.