By default, QuickTime sets itself up to intercept tiff files, i.e., the mime type when you download is image/tiff.

To undo this association, launch QuickTime from the Control Panel, select edit preferences in QuickTime, select browser plug-in, then MIME-settings. Scroll down to Images, click '+' to expand the choices, unselect tiff and click OK.

After this, QuickTime will no longer interfere with tiff downloads.