Updated from r3786 to r3790 yesterday, testing attachment adding on several Windows computers with Safari 5.0 fails. The file selection window opens, but once you click to select the file, the upload button-box disappears and no attachment is shown. Works as expected in Safari for Mac.
Reverted to r3788 for now.
Here is report of this problem #1486823 (Attachments don't work on firefox) ? Roundcube Webmail (http://trac.roundcube.net/ticket/1486823)