I've been trying to upload a sprite sheet, but TSR keeps saying that my sheet needs to be a PNG, GIF or zip file. The problem is that it is a PNG file (and my computer registers it as a PNG file), but it just won't send. I've successfully uploaded two other sheets, so it's not my first time doing this. Any advice on how to get my sheet in?
Based on that error message, it seems that the file you are trying to upload is actually a BMP image that had its extension changed to .png which won't work.