Redmine

I'm a Newbie on redmine, I'm using redmine 2;

I'm developing a plugin, to upload assets to a Canto cumulus database via a rest-api.

How can I integrate the form in the issue - edit - form? I'm using a hook in "view_issues_form_details_bottom" an the form looks like this:

<%= form_tag(baseurl + '/asset/import/sample/empty', :method=>'post', :multipart => true) do %>
 <p>
   Hash:<br />
   <%= text_field_tag "fields" %>
 </p>
 <p>
   Attachment:<br />
   <%= file_field_tag "attachment" %>
 </p>
 <p>
   <%= submit_tag 'Submit' %>
 </p>
<% end %>

If I integrate the form in the issue - show - form it works.