You actually want to do this inside your view rather than your model and it's actually quite straight forward. We're just going to loop through the errors, and when the one for your attachment comes up we'll ignore the field name:
<ul>
<% @myObject.errors.keys.each do |field| %>
<% @myObject.errors[field].each do |msg| %>
<% if field == :image_file_name %>
<li><%= msg %></li>
<% else %>
<li><%= field.to_s + " " + msg %></li>
<% end %>
<% end %>
<% end %>
</ul>
Replacing @myObject
with the name of your model that should display only the message set to your attachment validation errors. This is just a simple example that displays them inline with the rest, but of course you could do anything you like with the messages. It's important to keep the name of the field that had the error in case you want to program any logic thats specific to its failure without having to rely on the error message staying exactly the same forever.