Safer Paperclip id_partition interpolation for mongoid

paperclip.rb

# Googling for how to implement a good Paperclip id_partition for mongoid, you might find -
# attachment.instance.id.to_s.scan(/.{4}/).join("/")

# There are problems with this. Because mongoid ID's are hex strings, a 4-character string can contain
# 4^16 or 65536 values. Usually with id partition we're trying to make sure no directory ever holds more
# than 32k files, so 64k is potentially dangerous. To be safe we need to drop to 3 characters or 4096 values.

Paperclip.interpolates :id_partition do |attachment, style|
  attachment.instance.id.to_s.scan(/.{3}/).join("/")
end