Fix bridging video attachments from matrix clients that didn't include an extension

[ownaginatious]

I'm running mautrix-imessage locally and noticed that when I send a video attachment from the Element iOS client, the video attachment arrives as an unknown file in the Messages app:

It would appear this is happening because the file is simply called Video without an extension. You would think the mime type would be enough to infer that, but I guess not ¯\_ (ツ)_/¯

This PR fixes it by adding the (probable) extension based on the mime type to correct the issue.