Preview of attachments in mkv files

[LeMoi]

Hello there,
I just discovered the Icaros tweak, but I couldn't manage to do what I want to: using attachments jpg as thumbnail for mkv files.
I checked the option in settings, but it doesn't work, every mkv file I own has 1 or 2 cover files, and the tweak doesn't allow to display any of them, it justs shows a preview from the video.
Is there something mandatory in the filename or any property of the attachment that would allow it to be used as thumbnail?
Thanks in advance for your help

[LeMoi]

I was told that the attached jpg has to be named cover.jpg...
Is it possible to add an option to use first jpeg file in attachments, instead of looking for the cover.jpg file? Thanks in advance!