test.rb

    bills = [
      {:name=>"bill[attachment]",
       :filename=>"/data/data/com.rhomobile.imageupload/rhodata/apps/rhoconfig.txt"},
      {:name=>"bill[amount]",
       :body=>nil}]

    Rho::AsyncHttp.upload_file( 
      :url => 'http://localhost:3000/bills.json',
      :callback => url_for(:action => :push_callback),
      :multipart => bills) 

# the problem is probaly here: https://github.com/rhomobile/rhodes/blob/59c19e246b5df732d9c9269648f366a0c5912e26/platform/shared/net/AsyncHttp.cpp#L138

# Zero length body makes rhodes switch to else case and set content-type.
# I believe it has to be all the other way:
#If (file)
#  set content type
# else # means body
#  blank content type
# end