Heroku's docs say to use Tempfile.
Well, it says in the doc:
There are two directories that are
writeable: ./tmp and ./log (under your
application root). If you wish to drop
a file temporarily for the duration of
the request, you can write to a
filename like
#{RAILS_ROOT}/tmp/myfile_#{Process.pid}.
There is no guarantee that this file
will be there on subsequent requests
(although it might be), so this should
not be used for any kind of permanent
storage
Then, if you click on the Adam Wiggins link below that, he says this is available through the Tempfile interface.
Using Tempfile is easy, but your file may not exist if heroku considers read/write file operations different processes.
# tempfile_example.rb
require 'tempfile'
# defaults to Dir::tempdir
x = Tempfile.new('imagefile.png')
puts x.path
Edit: The Answer
The answer is actually to set :path => " " ... but you were close.... – Angela