Add check for support letter attachments in form validation

app/assets/javascripts/frontend/form-validation.js.coffee

@@ -56,6 +56,9 @@ window.FormValidation =
   isCheckboxQuestion: (question) ->
     question.find("input[type='checkbox']").length
 
+  isSupportLetterAttachment: (question) ->
+    question.find(".js-support-letter-attachment").length
+
   toDate: (str) ->
     moment(str, "DD/MM/YYYY")
 
@@ -81,6 +84,9 @@ window.FormValidation =
       if @isCheckboxQuestion(question)
         return question.find("input[type='checkbox']").filter(":checked").length
 
+      if @isSupportLetterAttachment(question)
+        return question.find(".js-support-letter-attachment-id").val().toString().trim().length
+
   validateRequiredQuestion: (question) ->
     # if it's a conditional question, but condition was not satisfied
     conditional = true