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“We’re trying to use SWFUpload in Rails 3 by using Rack middleware to rewrite the session cookie passed in via request param. In a Rails 2.3.x your Rack middleware could clobber the session cookie and reset it before it hits your main app. However, this isn’t happening in Rails 3. Has anybody run into this before?”
The answer is that as of Rails 2.3.4, the session handler was moved to Rack middleware. So you need to make sure that your middleware manipulating the session is running before the Rails session middleware is run. You can do this with something like this:
Brian Racer has a good description of getting SWFUpload working with rails 2.3.4