Drag & drop feature doesn't work on some web pages

[helperman]

Drag & Drop feature works on https://send.firefox.com/
but doesn't work on https://send.firefox.com/login/
If D&D feature is featured in a web page it has to be supported.

Steps to reproduce:
https://youtu.be/iWSJzsZIs20

Expected: a file dropped successfully on any web page that announce this feature
Actual: the file is opened in browser

[dannycoates]

thanks, fixed here 55df061