When mails are sent via attachment, the attachment is displayed empty in both preview and opened window if

• the attached mail is written in text-only format and is signed and/or encrypted
• the attached mail has encrypted attachments

I have generated mails with all permutations of outlook formats (html, rich, text) * (un)signed * (un)encrypted and sent them via attachment in various ways. Sending the attachments signed and/or encrypted does not matter, neither if it's a single or multiple attachements. In all cases the empty display in the preview / opened window is the same. This behaviour can be seen e.g. in the following attachments of the mail Ted:INBOX/Anhänge/attached unsigned unencrypted: all mailformats (15:07):

• TEXT signed unencrypted
• TEXT unsigned encrypted
• TEXT signed encrypted

Additionally attaching mails with encrypted attachments (does not matter if signed or not) leads to the same behaviour, which can be seen in the following attachments of the mail Ted:INBOX/Anhänge/attached unsingned unencrypted: all attachment mails (16:30):

• attached unsigned encrypted: all mailformats
• attached signed encrypted: all mailformats

Notes:

• RICH/HTML and encrypted attachements are displayed with their own openpgp-encrypted-message.asc attachment, which in turn can be opened fine (although it's displayed via Kleopatra and only as text not html). Not sure, if this is the expected behaviour