The problem there is not "strong parameters" - or you get a "forbidden..." error. The problem is that Rails broke the save-blob-to-db functionality, at least with regard to how it worked in Rails 3. This upgrade to 4 ...ugh!
I gave a workaround here: Trouble with uploading file after upgrating rails from 3.2 to 4
You will have to "fix" this in your controller, by taking what your form returns - I am guessing "params[:image]" - and adding the ".read" to it like this:
@slide.image = params[:image].read
@slide.save
That assumes you are using "@slide" for your model-object instance. If your code above is trimmed down, you get the added joy of isolating, removing, and saving that one param separately, as you cannot save it with the others, anymore.
See here for the methods why: http://api.rubyonrails.org/classes/ActionDispatch/Http/UploadedFile.html