Added a validation to validate against the content-type

Added tests to prove that the new validation is working.
Added the public directory to .gitignore
Changed the upfile content_type code to assign txt files to text/plain mime type

.gitignore

@@ -2,3 +2,4 @@
 *.swp
 tmp
 s3.yml
+public
\ No newline at end of file

lib/paperclip.rb

@@ -37,6 +37,9 @@ require 'paperclip/attachment'
 module Paperclip
 
   VERSION = "2.1.0"
+  
+  @@content_types      = ['image/jpeg', 'image/pjpeg', 'image/gif', 'image/png', 'image/x-png', 'image/jpg']
+  mattr_reader :content_types
 
   class << self
     # Provides configurability to Paperclip. There are a number of options available, such as:
@@ -166,6 +169,22 @@ module Paperclip
         end
       end
     end
+    
+    # Places ActiveRecord-style validations on the content type of the file assigned. The
+    # possible options are:
+    # * +content_type+: Allowed content types.  Can be a single content type or an array.  Allows all by default.  Use :image to allow all standard image types.
+    # * +message+: The message to display when the uploaded file has an invalid content type.
+    def validates_attachment_content_type name, options = {}
+      attachment_definitions[name][:validations] << lambda do |attachment, instance|
+        options[:content_type] = [options[:content_type]].flatten.collect! { |t| t == :image ? Paperclip.content_types : t }.flatten unless options[:content_type].nil?
+        
+        unless options[:content_type].empty?
+          unless options[:content_type].include?(instance[:"#{name}_content_type"])
+            options[:message] || ActiveRecord::Errors.default_error_messages[:inclusion]
+          end
+        end
+      end
+    end
 
     # Returns the attachment definitions defined by each call to has_attached_file.
     def attachment_definitions

lib/paperclip/upfile.rb

@@ -9,7 +9,8 @@ module Paperclip
       type = self.path.match(/\.(\w+)$/)[1] rescue "octet-stream"
       case type
       when "jpg", "png", "gif" then "image/#{type}"
-      when "txt", "csv", "xml", "html", "htm", "css", "js" then "text/#{type}"
+      when "txt" then "text/plain"
+      when "csv", "xml", "html", "htm", "css", "js" then "text/#{type}"
       else "x-application/#{type}"
       end
     end

test/test_paperclip.rb

@@ -64,12 +64,15 @@ class PaperclipTest < Test::Unit::TestCase
       assert Dummy.new.respond_to?(:avatar=)
     end
 
-    [[:presence,   nil,               "5k.png", nil],
-     [:size,       {:in => 1..10240}, "5k.png", "12k.png"]].each do |args|
+    [[:presence,   nil,                                "5k.png", nil],
+     [:size,       {:in => 1..10240},                  "5k.png", "12k.png"],
+     [:content_type1,{:content_type => "image/png"},   "5k.png", "text.txt"],
+     [:content_type2, {:content_type => :image},       "5k.png", "text.txt"],
+     [:content_type3, {:content_type => "text/plain"}, "text.txt", "5k.png"]].each do |args|
       context "with #{args[0]} validations" do
         setup do
           Dummy.class_eval do
-            send(*[:"validates_attachment_#{args[0]}", :avatar, args[1]].compact)
+            send(*[:"validates_attachment_#{args[0].to_s[/[a-z_]*/]}", :avatar, args[1]].compact)
           end
           @dummy = Dummy.new
         end