As Thomas explained, a message forwarded with attachments will contain an attachment made up of the original message content. It will contain two such attachments if the original message was received in both a plain text version and an HTML version. One will be listed with file type "plain text" and the other with "HTML text". You can remove them before sending.
As Thomas explained, a message forwarded with attachments will contain an attachment made up of the original message content.  It will contain two such attachments if the original message was received in both a plain text version and an HTML version.  One will be listed with file type "plain text" and the other with "HTML text".  You can remove them before sending.