When using files whose file names do not end in 'rec' an error message may appear and processing will not begin.
There are two ways to work around this problem:
The first is to always use the suggested naming system outlined in HARP.
The second, if you do not wish to use the naming pattern, is to navigate to the 'Options' tab, and type *.tif (if you're using .tif files) or *.bmp (if you're using .bmp files) etc.