On Debian, the magic file is in /usr/share/file/magic.
---- roundcube-0.1/config/main.inc.php.dist~ 2008-02-21 11:27:19.000000000 +0100
-+++ roundcube-0.1/config/main.inc.php.dist 2008-03-06 14:04:53.000000000 +0100
-@@ -277,7 +277,7 @@
- $rcmail_config['max_pagesize'] = 200;
+Index: b/config/main.inc.php.dist
+===================================================================
+--- a/config/main.inc.php.dist 2009-10-21 10:47:40.000000000 +0200
++++ b/config/main.inc.php.dist 2010-07-17 17:33:23.000000000 +0200
+@@ -301,7 +301,7 @@
+ $rcmail_config['identities_level'] = 0;
// mime magic database
-$rcmail_config['mime_magic'] = '/usr/share/misc/magic';
+$rcmail_config['mime_magic'] = '/usr/share/file/magic';
- // end of config file
- ?>
+ // Enable DNS checking for e-mail address validation
+ $rcmail_config['email_dns_check'] = false;