The File Extension Library can help you to figure out just what type of file that is you have. Their extensive list of file endings can be clicked on for better explanation of just what that particular extension is for and what purpose it serves. For example what is a dmg file extension? I actually didn't know until I went to the File Extension Library to find out. "The dmg file extension is a Mac OS X Copy Disk Image File that falls in the category of Disk Files. It is a mountable disk image file that is a creation of Mac OS X, which normally opens image files mounting virtual disks on the desktop. This file format is a replacement of .IMG file extensions normally used by MAC OS Classic. Mac OS 9 and all the earlier versions are not compatible with this file. DMG disk images are opened on Mac computers by use of Apple Disk Utility normally packaged in Mac OS X.
There is also a handy tool there to run to see if you have any errors! Remember though that The DMG file extension is Mac-specific and is not intended to be opened using Windows Operating system. However, users can make use of Daemon Tools in mounting this file’s images on virtual drivers. There is helpful information on what to do if you can't open dmg's. How great to have all this information in one easy to use Library where the answer is just a click away!