Describe workaround for false spoofing errors

As described in issue #1470, Paperclip can sometimes raise undesired spoofing errors when working with custom extensions. This adds an example workaround in the README using the public API of the mime-types library that will resolve this issue in a way compatible with Paperclip.
Merge pull request #1602 from norman/patch-1

Describe workaround for false spoofing errors