strange enough, in the message sent from sogo I see:

------=_=-_OpenGroupware_org_NGMime-27701-1492702673.011745-19------
Content-Type: application/pdf
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="Guida_modifica_firma.pdf"
Content-Length: 126322

while in the one sent from roundcube:

--=_9edf75b1710d8931606dd7c1a3011b48
Content-Transfer-Encoding: base64
Content-Type: application/pdf;
name=Guida_modifica_firma.pdf
Content-Disposition: attachment;
filename=Guida_modifica_firma.pdf

OK, did some more testa
The issues is in some way related to the signature
Sent a mail from an account withouthtml signature and the attachment is correctly display ed

In my case signature is composed by an embedded IMG (in HTML, I set IMG SRC and the base64 encoded image as source)

Some more tests
Created a signature in roundcube with embedded image and tried again
No issues at all, so it seems definitely a sogo only problem

any hint to debug this one?

How is connected your iPad?

Standard IMAP account? ActiveSync? Using SOGo's web interface?

messages are sent from SOGo web mail

messages are read (on iPad) via default mail client connected in imap to a google account

the recipient ISP doesn't affect the result: everytime I send an email from SOGo web mail with the embedded html signature and an attachment, the recipient can't see the attachment in his mail client on IOS (tried on iPad, iPhone ecc.. many free isp like google or libero.it)

the same email, viewed on a different client (outlook via imap, TB via imap or on remote webmail) has no issues..

but an identical email (same embedded html email, same attachment, same text), sent from roundcube from the same server to the same recipient doesn't show any issue.. signature and attach are both visible