Ensure case-insensitive file extension comparison

Ensure that when generating the list of acceptable file extensions, that the list is lower cased, because the extension from the filename will be lower-cased before attempting to find the extension in the list of acceptable extensions.
Merge pull request #1549 from greezybacon/issue/fileextension-ci

Ensure case-insensitive file extension comparison

Reviewed-By: Peter Rotich <peter@osticket.com>