Mask image/x-png as image/png
Internet Explorer uploads PNG files with content type image/x-png, while the official content type is image/png. This is no big deal but Google Chrome doesn't interpret image/x-png as a PNG image and downloads it as if it were an octet stream.
A proposed patch is attached. I've modified the attachments controller to report image/x-png as image/png when serving attachments so that Chrome correctly shows the image inline.
This code could also have gone into the upload part, but I felt it was a bad idea to modify the database for this. There's a test included but I couldn't easily get the tests to run so it is untested.
Based on trunk r2792.