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().