File field fails to add valid extension dm_73

This seems to be down to a regex in file.field.inc line 146
if (!preg_match('/^([a-z0-9]+([.][a-z0-9])* ?)+$/', $extensions)) {

Is there a way i can override this behavior without hacking code? I was able to upload files with this extension in Drupal 6, and need the same functionality in order to upgrade our site.

Ok the only option i have had (as far as i can tell) in order to get my site working as it should, is to hack the core :(

By changing line 146 of /modules/file/file.field.inc from:
if (!preg_match('/^([a-z0-9]+([.][a-z0-9])* ?)+$/', $extensions)) {
to:
if (!preg_match('/^([a-z0-9_]+([.][a-z0-9_])* ?)+$/', $extensions)) {

I can now add quake demo files in my file field, im pretty certain that the regex is not as efficient/tight as it could be ill delve into that at a later point.