file_scan_directory's $mask should really be case insensitive (eregi)

file_scan_directory's $mask is case sensitive (it uses ereg()). It should really be case INsensitive, so that when searching for '.htm', files with extensions of HTM, htm, Html, HTML, etc. are all returned. However, this does means that if using a $mask of 'a', then both 'a.txt' and 'A.txt' will be returned (in other words, remember that this effects the search behavior *always*,not just when searching with an extension).

Regardless of the drawbacks, I think this is a change worth considering.