Sometimes I see the missing plug in mesage flash. then when the attachment finishes downloading it appears.
Also sometimes when I close the email and reopen it, the attachment is there.
if its mainly PDF files thst are not appearing, you could install the free version of adobe acrobat reader. That will put a plugin for PDF's in the proper place for inline viewing in mail
make sure under mail pereferences --> in the viewing tab you have the checkbox
display remote images in HTML messages( checked on)