Lupp, the title of the post “Read only content cannot be changed. No modifications will be accepted.” is the exact text of the error dialog. It has nothing to do with write-protected files, email attachments stored in /tmp, and the only “solution” out there is to delete the “protected” text. It is a known bug since 2011
and the correct answer to this post is "this is a known bug since 2011 and it is still marked as “NEW”.
I have the same issue.