PGP content not displayed

PGP full-encrypted emails (sender uses outlook with GPG4win) are not shown properly in mail app. In fact no content is shown at all. But when using MacOS Mail and GPG Suite it works (encrypted.asc file is shown properly in mail app). I know you might say, this sounds like an Outlook issue regarding dealing with appended files. But could you display at least the mailbody in mail app? I am using GPG Applet so I just need to copy & paste the text-content.

I am using latest Nextcloud version on latest Firefox. No plugins are used in latest Firefox except uBlock Origin, but turned off.