keeping file field if error

hi, i'm doing a plugin for upload files, but after upload i have to
validate some fields and if don't pass de validation come back to the
create page keeping the introduced dates, so the user can change the
wrongs date without itroduce all again, my problem is that i can keep
all the dates less the file, and i'd like keep it too, any idea?

the code of the controller for keep dates:

def new

    @document = AlfrescoDocument.new(params[:document])
    @document.project_id = @project.id
    @edit_metadata = false
    @file=Tempfile.new(params[:file])
    @metadata = AlfrescoMetadata.new(params[:metadata])
    .....

and in the view the file is loaded with:

<p>
  <label><%=l(:label_attachment)%><span class="required">*</label>
  <%= file_field_tag 'file', :size => 30, :id => nil -%>
</p>

thanks.