Multi-documents upload fails when using Firefox against 5.2.f #121

When the user tries to upload multi-documents at once using drag and drop, only the last document in the list is successfuly uploaded.

Nothing happens for the others, even though the uploader dialog shows all green.
No errors are show in the logs.

I am relating this with 5.2.f especificaly because there was a problem with previous Share versions in the 5.2 branch that was making the multi-upload impossible, as described in the following issues:
https://issues.alfresco.com/jira/browse/MNT-18409
https://issues.alfresco.com/jira/browse/MNT-17940