Changed guess encoding to short circuit to binary if the mime type is…

… unknown, should be safe

lib/mail/attachments_list.rb

@@ -69,8 +69,14 @@ def []=(name, value)
       @parts_list << Part.new(hash)
     end
 
+    # Uses the mime type to try and guess the encoding, if it is a binary type, or unknown, then we
+    # set it to binary, otherwise as set to plain text
     def guess_encoding
-      @mime_type.binary? ? "binary" : "text"
+      if @mime_type && !@mime_type.binary?
+        "7bit"
+      else
+        "binary"
+      end
     end 
 
     def set_mime_type(filename)

spec/mail/attachments_list_spec.rb

@@ -63,6 +63,14 @@ def check_decoded(actual, expected)
                                         :mime_type => "application/x-gzip" }
       @mail.attachments[0].mime_type.should == 'application/x-gzip'
     end
+
+    it "should allow you to override the mime_type" do
+      file_data = File.read(filename = fixture('attachments', 'test.png'))
+      @mail.attachments['invoice.jpg'] = { :data => "you smiling",
+                                           :mime_type => "image/x-jpg",
+                                           :transfer_encoding => "base64" }
+      @mail.attachments[0].mime_type.should == 'image/x-jpg'
+    end
 
   end