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This can sometimes avoid dropping the wrong messages if syncing chats
takes too long.

WhatsApp uses the "title" and "fileName" fields for document names. The bridge
was only reading title and sending fileName, so sending a document from one
bridge to another through WhatsApp would lose the file name.

Also use names like "image.png" instead of "<random id>.png" for unnamed files

Set RestrictMessageSending in portal create+update

Fixes #209
Closes #233