Selecting file twice does not work in workflow

Following to ELEMENTS-748, it is now possible to attach a file to a workflow.

But attaching twice the same file does not work.

For instance, the following 3 tasks scenario shows the issue:

1. create a workflow with tasks
2. add a workflow variable
3. make it available and editable in each of them
4. set assignee to administrators group
5. start nuxeo
6. sync the project
7. create a document
8. start the workflow
9. attach a file ABC (task 1)
10. observe the filename now appears
11. click the X button
12. observe the filename has disappeared
13. attach the same file ABC (task 1)
14. observe the filename does not appear any longer and the upload button is now blue
15. approve the task
16. observe in task 2 the file does not appear

Expected behavior: attaching the same file twice should work.
Workaround: attach another file then the ABC file again and it works fine.