I have a message sent by Georg. It has 3 attachments of total size 7MB. The message size is 19 MB. Expected size would be below 10MB. Here's how looks the message structure:
User-Agent: KMail/4.13.0.8 (Linux/3.10.0-327.4.5.el7.x86_64; KDE/4.14.3; x86_64; Content-Type: multipart/signed; boundary="nextPart3645178.pEQGFe39hd"; micalg="pgp-sha1"; protocol="application/pgp-signature" --nextPart3645178.pEQGFe39hd Content-Type: multipart/mixed; boundary="nextPart41611280.VgUu2geZTb" Content-Transfer-Encoding: quoted-printable <here the QP-encoded message structure before signing - including attachments and text/plain part> --nextPart41611280.VgUu2geZTb Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="iso-8859-1" <here's again the text/plain part and attachments, finally signature>
So, as you see attachments are duplicated. The structure should be:
multipart/signed multipart/mixed (no content encoding here!) text/plain application/pdf ... application/pgp-signature