This problem mainly occurs when Microsoft Access is also installed in your computer. As Microsoft Access registers .mat file extension as a known file type, and the Windows Explorer folder settings for the computer are set to hide file extensions for known file types.

To make MAT files available again, perform one of the following procedures. Do not worry as it will not effect the functionality of Microsoft Access.
In Windows Explorer tools menu click Folder Options and clear hide file extensions for known file types check box, this works only with systems running in Windows 98 and 2K, where as for systems running in Windows NT do the following, in windows explorer on the view menu click options and on the view tab clear hide file extensions for known file types check box.