What I noticed is that even after increasing the PHP memory Size, it had no effect. I did see though, that this problems happens only when I send a mail that in the BODY contains a Signature using Apple mail (I have not tried with other clients) to send the mail
If I do not ad a signature, no matter how many or what size of attachment (within PHP_memory_limit), they can always be seen in roundcube
Another note. Although you cannot see them in roundcube, once I downoad the mail with Apple Mail again, the attachments are there, so is not that is not present, it's a display issue in roundcube. will try other mail clients to see if is APPLE MAIL or a general Interpretation error in mails with signatures and atachments added at the end in roundcube.