render :partial で別のフォーマットを使いたい時はfomats

formatで指定しても駄目だったので調べた。
ドキュメントに載ってなかったので、該当のメソッド読んだ結果

actionview-5.0.2/lib/action_view/renderer/partial_renderer.rb

    # Sets up instance variables needed for rendering a partial. This method
    # finds the options and details and extracts them. The method also contains
    # logic that handles the type of object passed in as the partial.
    #   
    # If +options[:partial]+ is a string, then the +@path+ instance variable is
    # set to that string. Otherwise, the +options[:partial]+ object must
    # respond to +to_partial_path+ in order to setup the path.
    def setup(context, options, block)
      @view   = context
      @options = options
      @block   = block

      @locals  = options[:locals] || {}
      @details = extract_details(options)

      prepend_formats(options[:formats])

      partial = options[:partial]
      (略)

formatsでOKっぽい。

<%= render :partial => "hoge", :formats => :js %>

と書けば良いっぽい。

actionview-5.0.2/lib/action_view/renderer/abstract_renderer.rb

    def prepend_formats(formats)
      formats = Array(formats)
      return if formats.empty? || @lookup_context.html_fallback_for_js

      @lookup_context.formats = formats | @lookup_context.formats
    end

内部的にprepend_formats内で最初にArrayしてるのでオプション自体は配列でもそうでなくても良い様です。