[imp] attached forward is deleted too early

[Martin Kleinschmidt]

Webmail (imp) 6.1.7 

when I forward a message as attachment, I get no error as the user, but
the attachment is missing.

In the log, I find
HORDE: [imp] PHP ERROR: filesize(): stat failed for /tmp/impatt77e1ay [pid 6391 on line 76 of "/home/horde4/imp/lib/Compose/Attachment/Storage/VfsLinked.php"]

This file does not exist during message composing.

If I change 

        $atc_file = Horde::getTempFile('impatt');

to

        $atc_file = Horde::getTempFile('impatt', false);

in /home/horde4/imp/lib/Compose.php, public function addAttachmentFromPart

everything works OK.

So obviously, the temporary file is deleted too early, but I don't know
how to track down why that is (I don't "speak" php...)

in /usr/share/php5/PEAR/Horde/Util.php, static public function shutdown,
the comment says:
"Deletes registered files at request shutdown." but what is a "request
shutdown"?