What are the point of 'm' graphics?

[Aflean]

I've been poking around the folders and found 'm' graphics. I've attempted to search for them but it came up with no results (although it's probably because I'm too shallow and/or I didn't use the right search query.) What are they?

[ElectriKong]

Files that give the GIF files transparency.

[Eri7]

"M" stands for masks if you didn't know.