[BitBucket user: Alain Mazy]
[BitBucket date: 2018-12-27.15:23:56]

Just created an independent upload page in plain HTML/JS: https://hg.orthanc-server.com/orthanc/changeset/e4f146b7d2d9903354f5c1feda6059e9c60ba168

If using the "browse" button, all files are always uploaded even when selecting 150 of them.
If using drag and drop, with a few files, it usually works but, as soon as you try to drop 150files, it takes firefox around 10 seconds to trigger the "dragover" and "drop" events and not all files that are dropped are listed in the events.  

This has been tested on Ubuntu 18.10 with Firefox 64.0.  I can not reproduce it on Windows.  

To me, the problem lies in the interaction between the Ubuntu file manager and Firefox.
On Ubuntu, I've tried uploading the same files to WeTransfer and Firefox is equally unresponsive when I drop 150 files.
So it looks like the only sure way to upload files via Firefox on Ubuntu is using the "browse" button (unfortunately, it's also incredibly slow and sometimes need 10 seconds to open the file browser dialog).

So it looks like upgrading jQuery ... was not necessary at all since the plain HTML/JS fails the same way.  I would recommend that:
- we forbid drag-drop for Firefox
- we add a "browse" button for all browsers
- instead of showing the issue #21 popup, on Firefox, we display a jQuery warning to tell users that drag-drop is not available

to be discussed ...