I currently have a form like this:

<% form_for @stem, :html => {:multipart => true} do |f| %>
<%= f.file_field :sound %>
<% end %>

This outputs (essentially):

<form method="post" id="new_stem" class="new_stem" action="/stems">
<input type="file" size="30" name="stem[sound]" id="stem_sound">
</form>

However I'm planning to use jQuery's ajaxForm plugin here and would like the new stem to be returned in JSON format. I know if the form's action was "/stems.json" this would work, but is there a parameter I can put into the form_for call to ask it to return JSON?

I tried doing

<% form_for @stem, :html => {:multipart => true, :action => '/stems.json'} do |f| %>

but this didn't appear to work.