Due to some mysterious "goings-on" I've begun to wonder about this. Student tracking shows that a file was uploaded, and its name was the file the student said he attached to his assignment submission, but the file upload shows it occurred after the "assignment submitted" was logged in the Action column. Successful assignment submissions are detailed in student tracking with the file upload(s) occurring before "assignment submitted" is logged. Other than that they look pretty similar. In the Tool column "File Manager" is listed in tracking next to File Uploaded in the Action column, but you can't find any files submitted by students in the Assignments Tool in File Manager.
(FYI - and, you can't find it attached to the assignment submission in this case either)