Remove BCF thumbnail from default form configuration

(cherry picked from commit 904f709ea3)

app/models/type/attribute_groups.rb

@@ -165,20 +165,25 @@ module Type::AttributeGroups
     end
   end
 
-
   ##
   # Get the default attribute groups for this type.
   # If it has activated custom fields through +custom_field_ids=+,
   # it will put them into the other group.
   def work_package_attributes_by_default_group_key
     active_cfs = active_custom_field_attributes
-
     work_package_attributes
       .keys
-      .reject { |key| CustomField.custom_field_attribute?(key) && !active_cfs.include?(key) }
+      .select { |key| default_attribute?(active_cfs, key) }
       .group_by { |key| default_group_key(key.to_sym) }
   end
 
+  ##
+  # Custom fields should not get included into the default form configuration.
+  # This method might get patched by modules.
+  def default_attribute?(active_cfs, key)
+    !(CustomField.custom_field_attribute?(key) && !active_cfs.include?(key))
+  end
+
   def to_attribute_group_class(groups)
     groups.map do |group|
       attributes = group[1]