[Mosu]

Do you have write access to the file you're trying to add as an attachment? Both tabs do very, very different things. The input tab will have mkvmerge scan the file for known container formats. On the attachment tab mkvmerge itself is not involved when adding a file, instead libMagic is called for automatic MIME type detection.

Can you provide one file for which the GUI crashes? I sure as hell cannot reproduce it here at the moment. I can add as many attachments as I want to, the GUI happily continues running.