Accept multipart upload with Sinatra

api.rb

post "/shots" do
  if params[:photo] && params[:photo].is_a?(Hash)
    class UploadedFile < OpenStruct; end
    paperclip = UploadedFile.new(
      tempfile:          params[:photo][:tempfile],
      path:              params[:photo][:tempfile].path,
      original_filename: params[:photo][:filename],
      content_type:      params[:photo][:type],
      head:              params[:photo][:head]
    )
    shot = @event.shots.new.tap do |s|
      s.photo = paperclip
    end
    shot.save!
    status 201
    headers "Location" => shot.api_url
    json shot.api_authenticated_hash
  else
    halt_with_400_bad_request("You need to provide a photo to upload")
  end
end