gistfile1.txt

<%= form_tag selected_categories_path, :multipart => true do %>
<%= text_field_tag 'dog[]', "dog1" -%>
<%= text_field_tag 'dog[]', "dog2" -%>
<%= file_field_tag 'attachment' -%>

 Parameters: {"dog"=>["dog1dog2"]}

gistfile2.txt

<%= form_tag selected_categories_path, :multipart => false do %>
<%= text_field_tag 'dog[]', "dog1" -%>
<%= text_field_tag 'dog[]', "dog2" -%>
<%= file_field_tag 'attachment' -%>

OR

<%= form_tag selected_categories_path, :multipart => true do %>
<%= text_field_tag 'dog[]', "dog1" -%>
<%= text_field_tag 'dog[]', "dog2" -%>


 Parameters: {"dog"=>["dog1", "dog2"]}