Hi Ash
I don't think I would actually attach something as attachments can make people click off an email. I would sketch the bare bones of your proposal into the body of the email, stating that you have 'x' idea, am thinking of approaching this way and was wondering if this prof. would be interested in talking to you in more detail about it. If you don't know the person you are writing you could give a BRIEF summary of your MSc project and then say how you see your PhD extending this, if at all. If you do know them, it's probably enough to remind him/her (in a line or two) of your MSc topic.
Good luck :-)