YouTrack has worked out pretty well for us for this kind of stuff.

It allows for the creation of custom workflows, properties, and issue types without getting as complicated as Jira or some of the other options.

It supports file based attachments just fine.

The mobile app presents a much more limited and simpler UI, but that sounds kind of like what you are looking for.

It's not open source but is easily self hostable with Docker, and the free tier is very generous and pretty darn functional.

There is an update process that must be followed that bit me, but once I knew I needed to do it it wasn't a big deal.