Don't use : characters in file names

[Tyriar]

This file breaks checkout on WIndows: https://github.com/jerch/xterm-addon-image/blob/master/fixture/testimages/sixel%3A-

[jerch]

@Tyriar Eww - no clue how this happened, must have been the cat. Looks like a sloppy file commit, as it contains other files that should not be in the repo :smile_cat:

[Tyriar]

I was a little taken aback when I saw the error from git, never seen that before 🙂