no attachment send when a + is in the filename

I am sending mails with rules after a new node is created.
(The content type has 2 fields for uploading files. These files should be send as attachments.)
This works most of the time but i found a problem when there is a + in the filename.
Instead of sending the file as attachment i get a file "attachment.dat" with the path in it.
I found that the + is missing in the path but the uploaded file has the + in the filename.
I guess its not working because Rules or Mimemail dosn't find the file.