All media except for images failing to upload in admin media

I am having this exact same issue on admin v1.9.12.

Images dragged into the media field trigger a POST event. All other file types do not.

https://discourse.getgrav.org/t/page-media-only-accepts-images/11544/6

_Originally posted by @matthewbecker1990 in https://github.com/getgrav/grav-plugin-admin/issues/1737#issuecomment-590236525_

The report identifies the admin media field and observes that only dragged images trigger a POST; no files or tests are named. Start by reproducing the upload behavior for non-image files and tracing the admin media upload entry point; done means non-image files trigger the expected upload request as well.

Written by the indexing model from the issue text.