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.

README.md

@@ -347,6 +347,18 @@ being uploaded as JPEGs, but will not prevent GIFs from being uploaded with a
 .jpg extension. This validation will only add validation errors to the form. It
 .jpg extension. This validation will only add validation errors to the form. It
 will not cause Errors to be raised.
 will not cause Errors to be raised.
 
 
+This can sometimes cause false validation errors in applications that use custom 
+file extensions. In these cases you may wish to add your custom extension to the
+list of file extensions allowed for your mime type configured by the mime-types 
+gem:
+
+```ruby
+# Allow ".foo" as an extension for files with the mime type "text/plain".
+text_plain = MIME::Types["text/plain"].first
+text_plain.extensions << "foo"
+MIME::Types.index_extensions text_plain
+```
+
 Defaults
 Defaults
 --------
 --------
 Global defaults for all your paperclip attachments can be defined by changing the Paperclip::Attachment.default_options Hash, this can be useful for setting your default storage settings per example so you won't have to define them in every has_attached_file definition.
 Global defaults for all your paperclip attachments can be defined by changing the Paperclip::Attachment.default_options Hash, this can be useful for setting your default storage settings per example so you won't have to define them in every has_attached_file definition.