When I add a patch, I am able to flag it as maintainer approved but this does not work for patches that someone else has uploaded. I can approve a bug report that others create but not individual patches.
This is expected default Bugzilla behaviour, where objects (bugs/attachments/flags) can only be modified by the creator of those objects, not a bug. One can modify the maintainer-feedback flag because the Auto-Assigner automatically sets maintainer-feedback value to ? with the maintainers email as the value, allowing the bugzilla account with that email to change the flags value. We will be making changes in the future to have attachments follow the same behaviour. In the meantime, the attachment creator can manually set maintainer-approval ? <maintainer-email>, which will then allow you to approve it.
^Triage: reassign.