Drop mimemagic dependency

See thoughtbot#2678 for context

lib/paperclip.rb

@@ -64,8 +64,6 @@
   require "mime/types"
 end
 
-require 'mimemagic'
-require 'mimemagic/overlay'
 require 'logger'
 require 'cocaine'
 

lib/paperclip/content_type_detector.rb

@@ -60,17 +60,12 @@ def possible_types
     end
 
     def type_from_file_contents
-      type_from_mime_magic || type_from_file_command
+      type_from_file_command
     rescue Errno::ENOENT => e
       Paperclip.log("Error while determining content type: #{e}")
       SENSIBLE_DEFAULT
     end
 
-    def type_from_mime_magic
-      @type_from_mime_magic ||=
-        MimeMagic.by_magic(File.open(@filepath)).try(:type)
-    end
-
     def type_from_file_command
       @type_from_file_command ||=
         FileCommandContentTypeDetector.new(@filepath).detect

paperclip.gemspec

@@ -24,7 +24,6 @@ Gem::Specification.new do |s|
   s.add_dependency('activesupport', '>= 3.2.0')
   s.add_dependency('cocaine', '~> 0.5.5')
   s.add_dependency('mime-types')
-  s.add_dependency('mimemagic', '0.3.0')
 
   s.add_development_dependency('activerecord', '>= 3.2.0')
   s.add_development_dependency('shoulda')

spec/paperclip/io_adapters/abstract_adapter_spec.rb

@@ -14,7 +14,6 @@ def content_type
       @adapter = TestAdapter.new
       @adapter.stubs(:path).returns("image.png")
       Paperclip.stubs(:run).returns("image/png\n")
-      Paperclip::ContentTypeDetector.any_instance.stubs(:type_from_mime_magic).returns("image/png")
     end
 
     it "returns the content type without newline" do

spec/paperclip/io_adapters/file_adapter_spec.rb

@@ -77,8 +77,6 @@
         before do
           MIME::Types.stubs(:type_for).returns([])
           Paperclip.stubs(:run).returns("application/vnd.ms-office\n")
-          Paperclip::ContentTypeDetector.any_instance
-            .stubs(:type_from_mime_magic).returns("application/vnd.ms-office")
 
           @subject = Paperclip.io_adapters.for(@file)
         end