post_processing_bug.rb

#! /usr/bin/env ruby

require 'active_record' # 4.2.3
require 'paperclip' # 4.3.0

ActiveRecord::Migration.verbose = false
ActiveRecord::Base.establish_connection(:adapter => "sqlite3", :database => ":memory:")
ActiveRecord::Base.raise_in_transactional_callbacks = true
ActiveRecord::Schema.define(:version => 1) do
  create_table :widgets, force: true do |t|
    t.string   "image_file_name"
    t.string   "image_content_type"
    t.integer  "image_file_size"
    t.datetime "image_updated_at"
  end
end

class Widget < ActiveRecord::Base
  include Paperclip::Glue

  has_attached_file :image

  validates_attachment_content_type :image, :content_type => %r{image/.*}

  after_image_post_process :save_image_dimensions

  def save_image_dimensions
    puts "We should not get here if the image is invalid!"
  end
end

Widget.create! image: File.open('dummy_file.txt')

=begin
I, [2015-08-13T12:51:38.158976 #85028]  INFO -- : Command :: file -b --mime 'dummy_file.txt'
I, [2015-08-13T12:51:38.171773 #85028]  INFO -- : Command :: file -b --mime '/var/folders/9j/wmdwzgzs40n32kvsrtv183_m0000gn/T/73592beb6ee90d5f310a72375b18924120150813-85028-rvjtzo.txt'
We should not get here if the image is invalid!
I, [2015-08-13T12:51:38.201334 #85028]  INFO -- : Command :: file -b --mime '/var/folders/9j/wmdwzgzs40n32kvsrtv183_m0000gn/T/73592beb6ee90d5f310a72375b18924120150813-85028-1vbfe7v.txt'
`raise_record_invalid': Validation failed: Image content type is invalid, Image is invalid (ActiveRecord::RecordInvalid)
  ...
  from ./post_processing.rb:32:in `<main>'
=end