Note: My successful reproductions of this have all been on development machines with caching off, meaning it takes a while for the repository to load.
Steps to reproduce:

1. Go to a page with a form containing a file manager field (e.g. Add a File resource).
2. Click the "Add file" button on the File Manager
3. Select the "Upload a file" repository on the left
4. Reload the page
5. Click the "Add file" button on the File Manager
6. While the "Upload a file" repository is still loading, before the spinner disappears, quickly click the "Upload a file" repository on the left.
7. Click the "Attchement" field and select a file

Expectation:

• The file is attached to the form

Actual Result:

• The file input in the form is empty

This causes random failures with behat tests that upload files, as behat instantly and indescriminately clicks "Upload a file" as soon as the file picker is opened.