This is an unwanted side-effect of MDL-38555. Steps to reproduce:

1. Go to an upload form (e.g. https://qa.moodledemo.net/question/import.php?courseid=2 logged in as teacher).
2. Drag a large file into the file manager, and quickly click the "Import" button.
3. The "This page is asking you to confirm that you want to leave - data you have entered may not be saved." pop-up appears.
4. Click "Stay on page".
5. Wait for file upload to complete.

Expected result: you can now submit the form.

Acutal result: the "Import button is disabled".

As a developer, another way to reproduce this is to put a sleep(10) in lib/antivirus/clamav/classes/scanner.php. Saves constandly uploading large files.