Add a way to allow content_types that file misses

There are some file types that the file command doesn't understand.
Notably, files that have a format, but are ASCII-text based (like ruby,
certificates, etc), show up as text/plain. This presents a problem,
because the mime-types gem either doesn't report them at all (as for
.pem files) or reports them differently (as for .rb files).

This change allows a Paperclip.options[:content_type_mapping] hash,
which allows for a mapping of { :pem => "text/plain" } which tells the
spoof detector that this is an OK matchup and to allow it.

lib/paperclip.rb

@@ -82,7 +82,8 @@ module Paperclip
       :command_path => nil,
       :log => true,
       :log_command => true,
-      :swallow_stderr    => true
+      :swallow_stderr => true,
+      :content_type_mappings => {}
     }
   end
 

lib/paperclip/media_type_spoof_detector.rb

@@ -10,19 +10,44 @@ module Paperclip
     end
 
     def spoofed?
-      if ! @name.blank?
-        ! supplied_file_media_type.include?(calculated_media_type)
+      if @name.present? && media_type_mismatch? && mapping_override_mismatch?
+        Paperclip.log("Content Type Spoof: Filename #{File.basename(@name)} (#{supplied_file_content_types}), content type discovered from file command: #{calculated_content_type}. See documentation to allow this combination.")
+        true
       end
     end
 
     private
 
-    def supplied_file_media_type
-      MIME::Types.type_for(@name).collect(&:media_type)
+    def media_type_mismatch?
+      ! supplied_file_media_types.include?(calculated_media_type)
+    end
+
+    def mapping_override_mismatch?
+      mapped_content_type != calculated_content_type
+    end
+
+    def supplied_file_media_types
+      @supplied_file_media_types ||= MIME::Types.type_for(@name).collect(&:media_type)
     end
 
     def calculated_media_type
-      type_from_file_command.split("/").first
+      @calculated_media_type ||= calculated_content_type.split("/").first
+    end
+
+    def supplied_file_content_types
+      @supplied_file_content_types ||= MIME::Types.type_for(@name).collect(&:content_type)
+    end
+
+    def calculated_content_type
+      @calculated_content_type ||= type_from_file_command.chomp
+    end
+
+    def mapped_content_type
+      Paperclip.options[:content_type_mappings][filename_extension]
+    end
+
+    def filename_extension
+      File.extname(@name.to_s.downcase).sub(/^\./, '').to_sym
     end
 
     def type_from_file_command

test/media_type_spoof_detector_test.rb

@@ -25,4 +25,17 @@ class MediaTypeSpoofDetectorTest < Test::Unit::TestCase
     adapter = Paperclip.io_adapters.for(File.new(fixture_file("5k.png")))
     assert ! Paperclip::MediaTypeSpoofDetector.using(adapter, adapter.original_filename).spoofed?
   end
+
+  should 'not reject when the extension => content_type is in :content_type_mappings' do
+    begin
+      Paperclip.options[:content_type_mappings] = { pem: "text/plain" }
+      file = Tempfile.open(["test", ".PEM"])
+      file.puts "Certificate!"
+      file.close
+      adapter = Paperclip.io_adapters.for(File.new(file.path));
+      assert ! Paperclip::MediaTypeSpoofDetector.using(adapter, adapter.original_filename).spoofed?
+    ensure
+      Paperclip.options[:content_type_mappings] = {}
+    end
+  end
 end