As suggested in #879902: Disable upload widget for special cases, in some cases it's necessary to disable the upload widget in file fields—for example, to force users to upload using a certain content type and just reference files from a different content type. This module allows just that. It's still a bit rough around the edges, but it works. Requires FileField Sources.
Drupal 7 version
The Drupal 7 version is at a very early stage of development. Using it on production systems is not yet recommended.