ActiveRecord display different text for attribute on validation

What is the best way to tell rails to output a different name in the validation presented to the user than the actual attribute?

I have a field “trade_attached_pdf”

Which AR creates the error message “Trade attached pdf may only be in pdf format”

But I want the message to read “Attached pdf may only be in pdf format”

I found human_attribute_name in AR.base but is not clear how to implement. I have not done much with I18n, is that where I should perhaps be looking instead?

David