#17 warning if files found with different case

[glenneroo]

This might prove problematic if, for some odd reason, files from multiple sets/exports/cameras come together with different extensions.

Somehow show a list of files with different cases? Or at least warn the user and give them a list of all the variations.

If rename feature is ever added, possibly offer the option to auto-rename to all lower/uppercase for convenience.

Note: currently extensions are converted to upper-case for making an extension search.