- file_copy in includes/
file.inc - Copies a file to a new location. This is a powerful function that in many ways performs like an advanced version of copy().
- file_transfer in includes/
file.inc - Transfer file using http to client. Pipes a file through Drupal to the client.
- upload_file_download in modules/
upload.module - upload_form in modules/
upload.module - user_configure_settings in modules/
user.module - user_file_download in modules/
user.module - Implementation of hook_file_download().