form_tag versus form_for with :multipart => true

Consider the following two ways of doing forms, both with :multipart
=> true. Only
the form_for will work properly when using attachment_fu

<code>
<%= form_tag :action => 'create', :multipart => true %>
  <%= file_field 'assignment', :uploaded_data %>
  <%= submit_tag "Create" %>
<%= end_form_tag %>
</code>

HTML:
<code>
<form action="/assignment/create?multipart=true" method="post">
</code>

<code>
<% form_for :assignment, :url => {:action => 'create'}, :html =>
{:multipart => true}) do |f| %>
   <%= file_field :assignment, :uploaded_data %>
  <%= submit_tag "Upload Assignment" %>
<% end %>
</code>

HTML:

<code>
<form action="/assignment/create" enctype="multipart/form-data"
method="post">
   <input id="assignment_uploaded_data"
name="assignment[uploaded_data]" size="30" type="file" />
  <input name="commit" type="submit" value="Upload Assignment" />
</form>
</code>

Only the second form works with attachment_fu. Why are these two
translations incompatible?
I would expect the enctype to be specified on the first form above.

Thanks for any feedback,

  Gordon

the :multipart specification goes into the options hash for the
form_tag, such as the following:

<% form_tag({:action => 'settings'}, {:multipart => true, :class =>
"form-style2"} ) do -%>
...
<% end %>