Steps to reproduce:
1. Generate a dummy theme with the theme generator. You don't have to modify anything in it.
2. Start up a DXP 7.1 and deploy the theme.
3. Create a form with an Upload field.
4. Place it on a page through the Form portlet.
5. Visit the page URL and click on the Upload button to the right of the field.
Checkpoint: The file selector popup is opened.
6. Configure the page and apply the dummy theme to it.
7. Visit the page URL and click on the Upload button to the right of the field.
Result: Notice nothing happens, and there is an exception in the browser console:
Possibly related: https://issues.liferay.com/browse/LPS-96889
Attaching both dummy themes I used for testing (7.1 and master).
This is not reproducible on master due to commit https://github.com/liferay/liferay-portal/commit/9022cf2698206131e4d9b68228b517fe3f195307