Allow replacement of files with the same name

[joeng03]

Brief description of feature Currently, the tutor could not replace an old file with a new file of the same file name. As shown in the image above, the error message is also inaccurate as the old file has already been deleted. Tutors have to first delete the old file, reload the page, then upload the new file, resulting in a negative user experience.

Acceptance criteria

• [ ] Tutors can replace an old file with a new file of the same file name in one single upload operation.