Perple_X programs generate files that are differentiated only 
by a suffix (e.g., "pl" and "pl.dat"), this may be confusing 
with operating systems, such as Windows and OSX, that by default
suppress file suffixes in viewing programs such as Explorer.

Under Windows, to modify the default setting in open "My Computer", 
under the "tools menu" select "folder options" then press the 
"View" tab and deactivate the "Hide extensions for known file
types" option.