moodlelib MDL-19418 fix bugs introduced when converting clean_param to preg.

Aumusingly, one of the bugs was detected by a unit test that was in Moodle 1.9, but
which for some reason was not in HEAD. I have tidied up the clean_param unit tests
and added a few more.

lib/filelib.php

@@ -496,7 +496,7 @@ function mimeinfo($element, $filename) {
         $mimeinfo = get_mimetypes_array();
     }
 
-    if (eregi('\.([a-z0-9]+)$', $filename, $match)) {
+    if (preg_match('/\.([a-zA-Z0-9]+)$/', $filename, $match)) {
         if (isset($mimeinfo[strtolower($match[1])][$element])) {
             return $mimeinfo[strtolower($match[1])][$element];
         } else {