Move format validator check to application helper

app/helpers/application_helper.rb

@@ -1,4 +1,13 @@
 # frozen_string_literal: true
 
 module ApplicationHelper
+  def accepted_format_for_attachment(model_klass, attribute_name)
+    validator = model_klass.validators_on(attribute_name.to_sym).find do |v|
+      v.is_a?(ActiveStorageValidations::ContentTypeValidator)
+    end
+
+    return unless validator
+
+    validator.options[:in].map { |f| Mime[f].to_s }.uniq.join(", ")
+  end
 end

app/views/sites/_form.html.erb

@@ -71,7 +71,7 @@
       <%= f.label :delivery_map, class: "form-label" %>
       <%= f.file_field :delivery_map,
                        class: "form-control",
-                       accept: delivery_map_accepted_mime_types %>
+                       accept: accepted_format_for_attachment(@site.class, "delivery_map") %>
       <% if @site.delivery_map.attached? && @site.persisted? %>
         <%= image_tag @site.delivery_map.representation(resize_to_limit: [200, 200]), class: "ms-0 mt-2" %>
       <% end %>

spec/helpers/application_helper_spec.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe ApplicationHelper do
+  describe "accepted_format_for_attachment" do
+    context "with a model and attribute using ContentTypeValidator" do
+      it do
+        expect(helper.accepted_format_for_attachment(Site, "delivery_map")).to eq(
+          "image/png, image/jpeg, image/gif, application/pdf"
+        )
+      end
+    end
+
+    context "with a model using ContentTypeValidator (but not the attribute)" do
+      it { expect(helper.accepted_format_for_attachment(Site, "name")).to be_nil }
+    end
+
+    context "with a model not using ContentTypeValidator" do
+      it { expect(helper.accepted_format_for_attachment(Room, "name")).to be_nil }
+    end
+  end
+end