I think this is bug as far as the sequencer's advertised image formats: we register TIFF as a mime-type, but our 3rd party extraction code really only supports IFF, which is not the same thing: http://en.wikipedia.org/wiki/Tagged_Image_File_Format
Can you please open an issue for us to remove this from the sequencer's set of supported files ?
As far as supporting the (real) TIFF format, feel free to open an enhancement request. We can't really do it using the current ImageMetadata, we'd have to look at either a newer version of another library altogether (e.g. ImageMagick)
Here is some information about TIFF files, I hope it helps:
TIFF, originally called Tagged Image File Format, is a computer file format for storing images, including photographs, line art among graphic artists, the publishing industry, and both amateur and professional photographers in general.
TIFF format is supported widely in industry image processing applications, such as Photoshop (Adobe), GIMP (Jasc), PhotoImpact and Paint Shop Pro (Ulead) and Desktop publishing & Page Layout applications, like QuarkXPress and Adobe InDesign. Other applications, like scanning, faxing, word processing, OCR and more applications are also support TIFF format. You can choose a TIFF processing SDK whose way of processing is simple and fast to process TIFF files. I hope you success. Good luck.