According to https://snapcraft.io/docs/snap-format, the correct extensions for an icon are `.png` and `.svg`. However, if you use the .PNG extension (all caps), snapcraft still treats it as if it's a valid extension and copies it to /meta/gui/icon.PNG. If you then change the icon, to an svg file for example, and re-run snapcraft, it will not remove the `icon.PNG` file, instead /meta/gui/icon.svg and /meta/gui/icon.PNG will now both exist.