Drag and Drop to upload does not work on the Dev page

[SoftVision-CiprianMuresan]

[Notes]:

• The issue is not reproducible on the Stage and Prod pages.

[Affected versions]:

• Firefox Release, Beta, Nightly
• Chrome

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Steps to reproduce]:

1. Open the browser and navigate to https://send.dev.mozaws.net/.
2. Drag and drop a file in the drop area.
3. Observe the behavior.

[Expected result]:

• Upload starts.

[Actual results]:

• Nothing happens.

[Additional notes]:

• Here is a screen recording of the issue:
  

[SoftVision-CiprianMuresan]

The issue is no longer reproducible on the Dev page. Marking as verified.