My problem
Adding images and documents to require drag and drop from filemanager that supports this.
In my case I do not have a filemanager that supports this (I live in a browser and terminal), and this renders it impossible to upload files to Phabricator.

Possible solution
Having a click to upload in addition to drag and drop where files can be uploaded would solve this problem.
I would also argue that click to upload is much easier then drag and drop since it does not require you to have several windows open on the desktop.