Show supported attachment type's

At the moment i have no way to see if a file type is supported in the attachment upload, until i choose a wrong one during upload

**Describe the solution you'd like**
A simple list of supported types would be greatly appreciated

Start by tracing the attachment upload flow and its current file-type rejection behavior. Identify where supported types are defined, expose them as a simple list before upload, and verify that the list matches the types accepted during validation.

Written by the indexing model from the issue text.