Use filter instance method `available?` for checking Enterprise Token

app/models/queries/work_packages.rb

@@ -34,10 +34,8 @@ module Queries::WorkPackages
 
   register.filter Query, filters_module::AssignedToFilter
   register.filter Query, filters_module::AssigneeOrGroupFilter
-  if EnterpriseToken.allows_to?(:attachment_filters)
-    register.filter Query, filters_module::AttachmentContentFilter
-    register.filter Query, filters_module::AttachmentFileNameFilter
-  end
+  register.filter Query, filters_module::AttachmentContentFilter
+  register.filter Query, filters_module::AttachmentFileNameFilter
   register.filter Query, filters_module::AuthorFilter
   register.filter Query, filters_module::CategoryFilter
   register.filter Query, filters_module::CreatedAtFilter

app/models/queries/work_packages/filter/attachment_base_filter.rb

@@ -36,6 +36,11 @@ class Queries::WorkPackages::Filter::AttachmentBaseFilter < Queries::WorkPackage
     :attachments
   end
 
+  def available?
+    EnterpriseToken.allows_to?(:attachment_filters)
+  end
+
+
   def search_column
     raise NotImplementedError
   end