If you set a file upload assignment to allow multiple attempts:

  1. Canvas does not overwrite the files from previous attempts.

  2. In SpeedGrader, you can select (and download) the files from the earlier attempt(s), as shown in this screenshot.

    Latest attempt file upload (by default, Canvas will display the latest attempt):

    Previous attempt file upload (note the warning that you are not viewing the latest attempt):
  3. However, if you bulk download the students' uploaded files, only the files from the latest attempt of each student will be downloaded.