ok from what i notice so far is people are renaming the file wrong. Not all systems will for the format in the file name so when people type minecraft_1.2.5.jar while renaming it is renaming it to minecraft_1.2.5.jar.jar (including the format) as stated in a previous post by another user. just rename to minecraft_1.2.5 and this wont change the file format on most system setups.