I like Bitwarden and open source, but attachments are a paid feature for them, so you'd be essentially working to add value to their paid features for free. That feels unfair to me.
I think you’d be working to add quality to your backups for free. Sure, they’d also give it to other customers, but what matters is that you have it (and henceforth do not have to maintain your own fork).
Isn't this argument basically true of any major open-source project? There's always a commercial user somewhere who benefits from your work without paying for it.
A contribution to Bitwarden would benefit the paid hosting, sure, but it'd also benefit folks who are self-hosting.