Forms: file upload field — allow customize file type identifiers

Once we know what exact file formats we can allow, let's also add UI to control, at minimum, between:

- Images
- Video
- Audio
- Other

A potential good advanced addition would allow directly control `accepts` attribute.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/file#unique_file_type_specifiers

Start by locating the Forms file-upload field and reviewing the linked MDN guidance on unique file type specifiers. Clarify which file categories and whether direct accepts-attribute control are required, then verify that the UI exposes the agreed controls and applies the resulting identifiers.

Written by the indexing model from the issue text.