form_for vs remote_form_for

I currently have this form:

<% form_for :workout_file, :url => { :action => 'create' }, :html =>
{ :multipart => true } do |f| -%>
  <p><%= f.file_field :uploaded_data %></p>
  <p><%= submit_tag :Create %></p>
<% end -%>

At the end of the action create I've got:

render :partial => 'workouts'

I'd like the partial to render inside a specific div, so I change the
form tag to:

<% remote_form_for :workout_file, :update => 'contentArea', :url =>
{ :action => 'create' }, :html => { :multipart => true } do |f| -%>

But that creates the following error from attachments_fu:

Thanx in advance!!

NoMethodError in WorkoutController#create

undefined method `content_type' for "/files/apps/polar/
07031301.hrm":String

RAILS_ROOT: script/../config/..
Application Trace | Framework Trace | Full Trace

vendor/plugins/acts_as_attachment/lib/technoweenie/acts_as_attachment/
instance_methods.rb:56:in `uploaded_data='
/usr/lib/ruby/gems/1.8/gems/activerecord-1.15.1/lib/active_record/
base.rb:1660:in `attributes='
/usr/lib/ruby/gems/1.8/gems/activerecord-1.15.1/lib/active_record/
base.rb:1659:in `attributes='
/usr/lib/ruby/gems/1.8/gems/activerecord-1.15.1/lib/active_record/
base.rb:1493:in `initialize_without_callbacks'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.15.1/lib/active_record/
callbacks.rb:225:in `initialize'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.15.1/lib/active_record/
validations.rb:726:in `create!'
app/controllers/workout_controller.rb:94:in `create'