Renaming the extension doesn't ruin the file/image format. Window's relies strongly on extensions to know which program use to ipen a certain file when you double click it. Other OSes, like Mac and Linux use the magic number to determine the type of file. If your orogram is able ro open the file once renamed, you may simply right click on one of them, with the original extension, choose "open with" the program you use and tick the box to always use that program. This should solve your problem.