Mail.app - Moving email with attachment to a subfolder causes the email to convert to text format and lose the attachment

I am Lion on Mac Book pro.

When I move a mail with attachement to a folder, sometimes, it loses its mime-type and becomes a text formatted message. Hence the contents of the attachments are added as text to the mail and I cannot download the attachments then.

This does not happen always, but almost 3/10 times.

Thanks.

Regards,

Viraj