Add a custom mime part header that specifies a URL for the attachment. This mime header is used by Thunderbird's RSS reader
and eventually by the code to handle storing attachments separately from the message.
Removes an obsolete attachment attribute called notDownloaded and replaces it with isExternalAttachment.
sr=bienvenu
a=sspitzer
git-svn-id: svn://10.0.0.236/trunk@169194 18797224-902f-48f8-a5cc-f745e15eee43
1b42dd2a1d
