this thread suggests trying to set the prefered format to html always to avail the issue. it kinda sorta worked on some cases for me. I am still loosing attachments but less frequently
Permanent solution:
Quit Mail
Open Terminal,
Enter "defaults write com.apple.mail PreferPlainText -bool FALSE"
Open Mail.
second issue i have is when sending attachments, my friends often see an extraneous blank attachment. The solution for it seems to me to check the following options
Edit->Attachment-> Send windows Friendy Attchment
Edit->Attachment-> Insert attachment at the end of the message