Hi,
In addition to what @FroKrahDiin has mentioned above about the .enpassdbsync MIME type, you would also need to add the .enpasattach MIME type (also with text/plain) if you want your attachments to sync to your WebDAV server.
Once you've done this, you'll need to re-sync everything from your main device to your server, then reinstall the Enpass apps on all your devices in order to sort out your attachment syncings.
Hope this helps somebody