Normalize function used to get derivative filename

Hi,

A little patch to normalize the function to get derivative filename.

The exiting method used to determine the extension is a confusion between the different types of dots, that can be either a normal character in the name of the file, or the separator between the name and the extension of the file.

As there is a php function to determine the extension, pathinfo(), it's better to use it, as in the patch, because it's more standard.

Sincerely,

Daniel Berthereau
Infodoc & Knowledge management