I supose that Roundcube needs to differentiate between "application/octet-stream" with filename with PDF/JPG/PNG... file extensions and "application/octet-stream" with others.
Ideally that Roundcube replaces "application/octet-strem" with PDF/JPG/PNG.. by "application/pdf" or ".../jpg", or ".../png", etc because if I change "application/octet-stream" by "application/pdf" with a text editor Roundcube works fine and opens the attachment in a new window.
It would be nice fix this...