How to validate presence of an uploaded file in rails?

Question

I'm playing around creating a rails file uploader and have struck a problem that should have an obvious solution. How do I check that a file has been selected in my form and uploaded?

Here is my new.html.erb view

<h2>Upload File</h2>
<% form_for(@upload_file, :url => {:action => 'save'}, :html => {:multipart => true}) do |f| %>
    <%= f.error_messages %>
  <p>
    <%= f.label :file -%>
    <%= f.file_field :upload -%>
  </p>
  <p>
    <%= f.label :description %>
    <%= f.text_field :description %>
  </p>
    <p>
        <%= f.label :file_type %>
        <%= f.select :file_type, ["XML Data"] %>
    </p>
  <p><%= f.submit 'Upload File' %></p>
<% end %>

and here is my upload_file.rb model

class UploadFile < ActiveRecord::Base
  validates_presence_of :description
  validates_presence_of :file_type
  validates_presence_of :upload
  def upload=(upload_file_field)
    self.name = "#{Time.now.strftime("%Y%m%d%H%M%S")}_#{upload_file_field.original_filename}"
    File.open("#{RAILS_ROOT}/public/upload/#{self.name}", "wb") { |f| f.write(upload_file_field.read) }
  end
end

If I use this as shown here, the validation validates_presence_of :upload always fails and I am returned to my form with an error message.

I'd be very grateful if someone could explain how to do this validation correctly, and I'd be even more grateful if they could explain why it works.

Thanks.