Why is Makefile always the gist's cover file and how to prevent it?

Question

I've learned to add the _ prefix in the file I want to be the cover of my gist, e.g.,

However, when my gist also have Makefile, for some reason, it becomes the cover file, e.g. (in this gist, I do have a file prefixed with _, but Makefile appears as the cover file)