MIddleware for making legacy versions of Internet Explorer (anything below 11) work with rails. I release this code under MIT license.

initializer.rb

# ./config/initializers/rewrite_ie_header.rb
# Don't forget to include the middleware file! If you have a ./lib/middleware folder in rails
# a `require './lib/middleware/rewrite_ie_header.rb'` should do the trick
#
# IE has a really stupid accept header, so here we scope its domain of mime-types that it can negotiate, otherwise we'll
# end up with weird bugs where if you hit /mcp/:id, IE will download a .ppt file instead of the .html content.
# 
# Issue documented in more detail at https://github.com/rails/rails/issues/9940.

Rails.application.config.middleware.use RewriteIEAcceptHeader do |ie_mime_types|
  ie_mime_types.register Mime::HTML
  ie_mime_types.register Mime::JSON
  ie_mime_types.register Mime::ALL
end

middleware.rb

# IE pukes a bunch of mime-types when we post forms at the server in a random order.
# so basically you can't count on those headers.
class RewriteIEAcceptHeader
  def initialize(app)
    @app = app
    yield self
  end

  def call(env)
    if crappy_browser?(env) && (mime_type = detect_mime_type(env))
      env["HTTP_ACCEPT"] = mime_type.to_s
    end
    @app.call(env)
  end

  def register(mime_type)
    mime_types << mime_type
  end

  private

  def mime_types
    @mime_types ||= []
  end

  def detect_mime_type(env)
    # PPT Presenter ajax calls don't have an ACCEPT header. However still have a MSIE mimetype.
    # Old Mime::Type in rails2 didn't care that this was nil. In Rails3 it explodes.
    Mime::Type.parse(env["HTTP_ACCEPT"] || "").find{ |m| mime_types.include?(m) }
  end

  # Detect if the user agent is IE trying to put or post...
  def crappy_browser?(env)
    env["HTTP_USER_AGENT"] =~ /msie/i
  end
end