hint the allowed image formats on favicon upload

app/helpers/favicon_helper.rb

+module FaviconHelper
+  def favicon_extension_whitelist
+    FaviconUploader::EXTENSION_WHITELIST
+      .map { |extension| "'.#{extension}'"}
+      .to_sentence
+  end
+end

app/uploaders/favicon_uploader.rb

 class FaviconUploader < AttachmentUploader
+  EXTENSION_WHITELIST = %w[png ico].freeze
+
   include CarrierWave::MiniMagick
   version :favicon_main do
@@ -11,7 +13,7 @@ class FaviconUploader < AttachmentUploader
   end
   def extension_whitelist
-    %w[png ico]
+    EXTENSION_WHITELIST
   end
   private

app/views/admin/appearances/_form.html.haml

@@ -70,7 +70,9 @@
       = f.hidden_field :favicon_cache
       = f.file_field :favicon, class: ''
       .hint
-        Maximum file size is 1MB. The resulting favicons will be cropped to be square and scaled down to a size of 32x32 px.
+        Maximum file size is 1MB. Allowed image formats are #{favicon_extension_whitelist}.
+        %br
+        The resulting favicons will be cropped to be square and scaled down to a size of 32x32 px.
   .form-actions
     = f.submit 'Save', class: 'btn btn-save append-right-10'