Attachments on transfers should show on both accounts

[Zegnat]

Currently when I have created a transfer, some of its data is synced on both accounts. E.g. changing its status or note is directly mirrored on both accounts. But when I add an attachment to it, this attachment is only available from one of the accounts

[mtotschnig]

@Zegnat Thanks a lot for bringing this up. I'll fix this for next version 3.9.2 at least for new transfers. I am hesitant: do you think existing transfers should be automatically fixed in the version's database upgrade?