If you want to delete just the attachments, there's no easy way from
the web interface. You can do a search for has:attachment, then for
each of the messages with one, forward it to yourself (Gmail will
remove the attachment), then delete the original.
However, If you use a desktop client and IMAP you might have more luck
because I think that most allow you to delete attachments. To figure
out how to set up IMAP, go to
http://mail.google.com/support/bin/answer.py?answer=77695
Ryan