This sometimes happens to me, but usually accompanies an alert that the current email has had images blocked because they reside on a remote server. When I click OK to allowing these images to download, suddenly the whole HTML email will appear. (I'm running under linux though, not that it should matter)
I'm also having a related problem:
When I receive a message that has attachments but that are embedded in the HTML body of the message, I can view it ok, but it doesn't show a little paperclip icon next to the message in my inbox. Its as though RoundCube doesn't recognise that there are attachments...
This is no big deal, but when I try to FORWARD the message, none of the attachments get forwarded.
Is this a bug or is there some way I can work/configure around it?