Increase the allowed file extension length for segmented images.
Increase the allowed file extension length for segmented images.

This commit increases the allowed file extension length
for segmented images from 3 to 4 characters.

It should be noted that the file extensions should be of a
consistent length for a set of segmented file otherwise
the sorting internally in various mounting methods (eg xmount)
will not work correctly and the resultant image will be corrupt.

Signed-off-by: Jason Dossett <e6f895edd39634df23774f0615e338f58a73f02c@utdallas.edu>

ldgriffin/imagemounter