Opening/Mounting a .img file How would I do this?

I have attempted double clicking on it (obviously) and I get an error of it saying that its not recognized.

My second though was using Disk Utility (the app) to try and change it from an img to an iso. The same error appeared saying that it was not recognized.

My last idea was to do some research on mounting from the terminal. The command that I used was 

mount -o loop userdata.img /mnt/testcache


The result of that was:

mount: You must specify a filesystem type with -t.


After that I just ran out of idea and ran out of search queries to try.
Provided the image is a correct image file supported by OS X the correct command to mount would be hdiutil attach imagename.

Not all files ending with .img are disk images at all, let alone supported by OS X. You can try to use file image name to see what this file actually might be, but even for valid classic image files this may just result in data as an answer.

It would be most helpful to know how or using which software this image was created.