Log Entire HTTP request (including body)

I have flash clients POSTing file uploads to a cluster fronted by
nginx. I see nginx logging 400 and 408 errors to these requests with a
high frequency and would like to inspect the requests to be sure Flash
isn’t sending malformed data.

What $http variable holds the entire request, and can I simply copy
the buffered file easily? If there isn’t a single variable can I just
log $http_request_headers and $http_request_body? I would setup a
specific location for 400 and 408s so not every request would be