Hi!
Thank you for sharing this workaround, but this really shouldn’t be the official way to handle it.
The Gallery field still has a “Allowed file types” (mime_types) setting option in the field configuration. It makes no sense for server-side validation to hardcode an image check while completely ignoring this field setting, or at least without providing a native filter to override it.
This completely breaks existing enterprise setups where galleries are used for document bundles (like PDFs, brochures, etc.).
I’m stuck on version 6.8.6 for now to avoid breaking client sites. I’ve also opened a ticket with support—hopefully, they will address this regression properly in an upcoming release.