That's a related thing I learned from that past project: file attachments are also extremely vulnerable to compatibility issues between CouchDB/PouchDB/Couchbase/Cloudant/etc. Even when they claim to support sizes large enough, they often fail to sync even when small. Attachments in general in the CouchDB protocol are a bottleneck that slows or effectively breaks sync if you have too many to sync at once, because most CouchDB-ish implementations tend to only process a single document at once among other factors.

The project I was working on involved regularly taking some photos and all the sync issues with attachments led to moving to syncing only metadata like width, height, and Blurhashes in the database itself and use a reference URL/pointer to a content-addressed storage (basically an S3 bucket-like with git-like hashes for filenames) we could background sync more directly outside of the CouchDB protocol. (Blurhashes were particularly useful there while waiting on that additional background sync phase after the CouchDB sync.)