Here's the solution for file input field.
To disable a submit button for file field when a file is not chosen, then enable after the user chooses a file to upload:
$(document).ready(function(){
$("#submitButtonId").attr("disabled", "disabled");
$("#fileFieldId").change(function(){
$("#submitButtonId").removeAttr("disabled");
});
});
Html:
<%= form_tag your_method_path, :multipart => true do %><%= file_field_tag :file, :accept => "text/csv", :id => "fileFieldId" %><%= submit_tag "Upload", :id => "submitButtonId" %><% end %>