Use tinymce from tinycme package instead of the shipped one.
---- roundcube-0.1~rc2.orig/program/steps/mail/sendmail.inc 2007-11-26 11:42:36.000000000 +0100
-+++ roundcube-0.1~rc2/program/steps/mail/sendmail.inc 2007-11-26 11:44:04.000000000 +0100
-@@ -123,7 +123,7 @@
+Index: roundcube/program/steps/mail/sendmail.inc
+===================================================================
+--- roundcube.orig/program/steps/mail/sendmail.inc 2012-09-29 11:30:58.259119253 +0200
++++ roundcube/program/steps/mail/sendmail.inc 2012-09-29 11:31:45.368061359 +0200
+@@ -118,7 +118,7 @@
// sanitize image name so resulting attachment doesn't leave images dir
$image_name = preg_replace('/[^a-zA-Z0-9_\.\-]/i', '', $image_name);