When a file is uploaded as part of a form submission, its MIME is included. I've discovered that Firefox on Windows has an inconvenient way of determining this MIME.
Using a comma in data can sometimes be tricky. WordPress taxonomies, I'm looking at you. Try this character instead.
A change in Google Fonts' URI structure for embedding combined fonts can lead to an incompatibility with WordPress.
How I resolved an incompatibility between WP Google Maps Pro and Autoptimize plugins preventing maps from initializing correctly.