FAILSAFE error when attempting custom POST - How do I fault find for this?

Hi,

I'm creating the file upload code in a C# web form to upload to my
Rails app. When I start the upload I get the following error on my
Rails server? Any ideas how I can fault find here?

It could be the case the HTTP post I'm creating has a problem (it'
multipart, with a file upload + some form parameters) however it's
hard to tell from Rails what it is causing it?

/!\ FAILSAFE /!\ Tue Jan 12 08:20:06 +1000 2010
  Status: 500 Internal Server Error
  bad content body
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/utils.rb:
351:in `parse_multipart'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/utils.rb:
323:in `loop'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/utils.rb:
323:in `parse_multipart'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/request.rb:
133:in `POST'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/
methodoverride.rb:15:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/actionpack-2.3.3/lib/
action_controller/params_parser.rb:15:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/actionpack-2.3.3/lib/
action_controller/session/cookie_store.rb:93:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/actionpack-2.3.3/lib/
action_controller/reloader.rb:29:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/actionpack-2.3.3/lib/
action_controller/failsafe.rb:26:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:
11:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:
11:in `synchronize'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:
11:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/actionpack-2.3.3/lib/
action_controller/dispatcher.rb:106:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.3/lib/rails/rack/
static.rb:31:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:
46:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:
40:in `each'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:
40:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.3/lib/rails/rack/
log_tailer.rb:17:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/
content_length.rb:13:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/chunked.rb:
15:in `call'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/handler/
mongrel.rb:64:in `process'
    /opt/local/lib/ruby/gems/1.8/gems/mongrel-1.1.4/lib/mongrel.rb:
159:in `process_client'
    /opt/local/lib/ruby/gems/1.8/gems/mongrel-1.1.4/lib/mongrel.rb:
158:in `each'
    /opt/local/lib/ruby/gems/1.8/gems/mongrel-1.1.4/lib/mongrel.rb:
158:in `process_client'
    /opt/local/lib/ruby/gems/1.8/gems/mongrel-1.1.4/lib/mongrel.rb:
285:in `run'
    /opt/local/lib/ruby/gems/1.8/gems/mongrel-1.1.4/lib/mongrel.rb:
285:in `initialize'
    /opt/local/lib/ruby/gems/1.8/gems/mongrel-1.1.4/lib/mongrel.rb:
285:in `new'
    /opt/local/lib/ruby/gems/1.8/gems/mongrel-1.1.4/lib/mongrel.rb:
285:in `run'
    /opt/local/lib/ruby/gems/1.8/gems/mongrel-1.1.4/lib/mongrel.rb:
268:in `initialize'
    /opt/local/lib/ruby/gems/1.8/gems/mongrel-1.1.4/lib/mongrel.rb:
268:in `new'
    /opt/local/lib/ruby/gems/1.8/gems/mongrel-1.1.4/lib/mongrel.rb:
268:in `run'
    /opt/local/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/handler/
mongrel.rb:34:in `run'
    /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.3/lib/commands/
server.rb:111
    /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in
`gem_original_require'
    /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in
`require'
    ./script/server:3
^CExiting

thanks