Thumbnail image for MKV files

[Steve01351]

I must be missing something, but I have no ide what it could be. I just want to make a thumbnail picture the display picture for an MKV. The directions I see say to just make sure the smaller size of the file is 600, rename the file cover.jpg, drag it into the attachments directory, and "Boom". It just doesn't work. I've been trying for a couple hours. I've tried "cover.jpg", coverart.jpg", and Cover Art.jpg". I've tried from the Multiplexer and from the Header Editor. Anyone have any idea what I'm missing?

I'm using a Windows 10 Pro computer.

Thanks

[Steve01351]

OK. I give up. I've been trying this all day. I've tried with different versions, I've tried running the program as an administrator, I've tried running in compatibility mode for Windows 8 - I've even tried installing and running it on a Linux machine. I drop the jpg into the attachments tab, I make sure the dimensions are 600x600, I make sure the filename is cover.jpg, but it just never takes. Frustrating.