repairing file extension
Here’s what wiki has to say about a file extension : A filename extension is a suffix to the name of a computer file applied to indicate the encoding convention (file format) of its contents.
In some operating systems (for example Unix) it is optional, while in some others (such as Windows) it is a requirement. Some operating systems limit the length of the extension (such as DOS and OS/2, to three characters) while others (such as Unix) do not. Some operating systems (for example RISC OS) do not use file extensions.
A lot of problems with file extension came from registry error. Now you can download a free software to find out if registry problem is really the problem, or you can search the type of your file extension and fix it with the steps provided there. Many types of file extension can be fix, for example file extension DMG, DLL, BIN,etc. I found it really helpful cause you can fix the file extension with easy steps.