Hi Pierre Igot,
here is some more new info. I can now remove the attachments, if I mark ALL messages in the folder 2012 but I cant remove after marking a single message that has an attachment.
And second, somewhere deep in the subdirectory, Mail creates a folder "Attachments". If you remove that it wont re-appear but you can still send mails with attachments. I have no idea where the attachments are stored then, maybe they are incorportated in the mails.
The file structure has change dramatically after the upgrade, could cause the problem.
Kind regards, Tino