They can create a text file called mime.txt in the folder where the SyncBackPro executable is. By default, this location is at
C:\Program Files (x86)\2BrightSparks\SyncBackPro
However, it may differ if you chose a different installation path during setup. Note that on 32-bit versions of Windows, the paths will likely be pointed to C:\Program Files\ instead.
You can open mime.txt to edit and specify the extension and what mime type it should be. Specify one extension/mime per line. For example:
Please note that it is not necessary to put a period before the extension (example - txt instead of .txt), but it wouldn't matter either if you place the period before such extensions.