Well.

Bit more of a trouble than I wish to deal with today, but apparently Mongrel has changed somewhat in the latest version and doesn't deliver content as HTML, instead delivering it as text/plain (source html).

If you are using Mongrel to deliver your Ruby Apps, and you're sure your versions of gems meet the requirements - take a look at the below to try.

Add this file in, restart your application - and hopefully away you go. Again there is a lot of read/take in when it comes to dealing with this issue - hopefully I'm saving you about 1.5 hours of research. Original Source for the Patch I've forked..

Add this to redmine/config/initializers/mongrel.rb (create a new file)

# Pulled right from latest rack. Old looked like this in 1.1.0 version.
  # 
  # def [](k)
  #   super(@names[k] ||= @names[k.downcase])
  # end
  # 
  module Rack
    module Utils
      class HeaderHash < Hash
        def [](k)
          super(@names[k]) if @names[k]
          super(@names[k.downcase])
        end
      end
    end
  end
  
  # Code pulled from the ticket above.
  # 
  class Mongrel::CGIWrapper
    def header_with_rails_fix(options = 'text/html')
      @head['cookie'] = options.delete('cookie').flatten.map { |v| v.sub(/^\n/,'') } if options.class != String and options['cookie']
      header_without_rails_fix(options)
    end
    alias_method_chain :header, :rails_fix
  end
  
  # Pulled right from 2.3.8 ActionPack. Simple diff was
  # 
  # if headers.include?('Set-Cookie')
  #   headers['cookie'] = headers.delete('Set-Cookie').split("\n")
  # end
  # 
  # to 
  # 
  # if headers['Set-Cookie']
  #   headers['cookie'] = headers.delete('Set-Cookie').split("\n")
  # end
  #       
  module ActionController
    class CGIHandler
      def self.dispatch_cgi(app, cgi, out = $stdout)
        env = cgi.__send__(:env_table)
        env.delete "HTTP_CONTENT_LENGTH"
        cgi.stdinput.extend ProperStream
        env["SCRIPT_NAME"] = "" if env["SCRIPT_NAME"] == "/"
        env.update({
          "rack.version" => [0,1],
          "rack.input" => cgi.stdinput,
          "rack.errors" => $stderr,
          "rack.multithread" => false,
          "rack.multiprocess" => true,
          "rack.run_once" => false,
          "rack.url_scheme" => ["yes", "on", "1"].include?(env["HTTPS"]) ? "https" : "http"
        })
        env["QUERY_STRING"] ||= ""
        env["HTTP_VERSION"] ||= env["SERVER_PROTOCOL"]
        env["REQUEST_PATH"] ||= "/"
        env.delete "PATH_INFO" if env["PATH_INFO"] == ""
        status, headers, body = app.call(env)
        begin
          out.binmode if out.respond_to?(:binmode)
          out.sync = false if out.respond_to?(:sync=)
          headers['Status'] = status.to_s
          if headers['Set-Cookie']
            headers['cookie'] = headers.delete('Set-Cookie').split("\n")
          end
          out.write(cgi.header(headers))
          body.each { |part|
            out.write part
            out.flush if out.respond_to?(:flush)
          }
        ensure
          body.close if body.respond_to?(:close)
        end
      end
    end
  end