When you upload files, they are automatically resized and converted to JPEG. But this is a copy of the file used for display on the web page. Enabling a public download of your file only grants access to a full resolution JPEG copy of your file.
Your original TIFF (or RAW or whatever format) is also stored online. But only YOU can download it in it's original format. This is done from the Photoshop Express Organizer (going to "My Library" on Photoshop.com).