Don't let JPEG images get saved with .png extension

Matti Viljanen requested to merge dont-save-jpg-as-png into master

If WF receives an attachment with a .png extension, it double-checks the MIME type from the contents of the file, and sets the extension to .jpg if necessary.

Fixes #374 (closed)

TODO: This is actually a bug in Signal Desktop, so it should be reported/fixed there, too.

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