diff --git a/.rubocop_todo.yml b/.rubocop_todo.yml
index 2fc81a28e..519f17fe2 100644
--- a/.rubocop_todo.yml
+++ b/.rubocop_todo.yml
@@ -390,7 +390,6 @@ Lint/HandleExceptions:
 
 Lint/IneffectiveAccessModifier:
   Exclude:
-    - 'app/models/attachment.rb'
     - 'app/models/mail_handler.rb'
 
 Lint/InterpolationCheck:
diff --git a/app/models/attachment.rb b/app/models/attachment.rb
index e5adbb236..afba75e60 100644
--- a/app/models/attachment.rb
+++ b/app/models/attachment.rb
@@ -492,21 +492,24 @@ class Attachment < ActiveRecord::Base
     time.strftime("%Y/%m")
   end
 
-  # Returns an ASCII or hashed filename that do not
-  # exists yet in the given subdirectory
-  def self.disk_filename(filename, directory=nil)
-    timestamp = DateTime.now.strftime("%y%m%d%H%M%S")
-    ascii = ''
-    if %r{^[a-zA-Z0-9_\.\-]*$}.match?(filename) && filename.length <= 50
-      ascii = filename
-    else
-      ascii = Digest::MD5.hexdigest(filename)
-      # keep the extension if any
-      ascii << $1 if filename =~ %r{(\.[a-zA-Z0-9]+)$}
+  # Singleton class method is public
+  class << self
+    # Returns an ASCII or hashed filename that do not
+    # exists yet in the given subdirectory
+    def disk_filename(filename, directory=nil)
+      timestamp = DateTime.now.strftime("%y%m%d%H%M%S")
+      ascii = ''
+      if %r{^[a-zA-Z0-9_\.\-]*$}.match?(filename) && filename.length <= 50
+        ascii = filename
+      else
+        ascii = Digest::MD5.hexdigest(filename)
+        # keep the extension if any
+        ascii << $1 if filename =~ %r{(\.[a-zA-Z0-9]+)$}
+      end
+      while File.exist?(File.join(storage_path, directory.to_s, "#{timestamp}_#{ascii}"))
+        timestamp.succ!
+      end
+      "#{timestamp}_#{ascii}"
     end
-    while File.exist?(File.join(storage_path, directory.to_s, "#{timestamp}_#{ascii}"))
-      timestamp.succ!
-    end
-    "#{timestamp}_#{ascii}"
   end
 end