Just a quick note for the people (like me) for seeing an error like this in their exception notifications and to save you some Googling:
ActionView::MissingTemplate: Missing template challenges/show with {:handlers=>[:haml, :rjs, :builder, :rhtml, :rxml, :erb], :locale=>[:en, :en], :formats=>["*/*;q=0.6"]} in view paths
This is an issue in MIME::Type.parse and has been fixed in Rails master 8 February, 2012 according to this pull request.
There is a patch in the comments on the pull request page you can use to fix it for your current app until a new Rails is released.