Quick hack, recently I needed to get the Rails Attachment_Fu image uploader to support TIFF & BMP image uploads. Took a fair bit to work out but in the end I hacked it by doing this.

First open the file...

/vendor/plugins/attachment_fu/technoweenie/attachment_fu/attachment_fu.rb

In this file look for @@content_types and add these to the end...

, 'image/bmp', 'image/tiff'

Quick & dirty but allows Attachment_Fu and RMagick to support BMP and TIFF uploads, along with all the rest.