I would definitely try to reach out. Your mark is incredibly important as you mentioned and it will affect your course (in which i’m sure you worked hard on!). Yes you made a mistake, but it was an accident and things happen. Universities aren’t unreasonable and they will understand that in a state of panic, you accidentally submitted a different file. What I would suggest you do is let them know as soon as possible. Be open and honest and tell them the entire story. I would also suggest you show them the actual piece of work you meant to submit so they can see that you are being honest, and not trying to plagiarize. Do this as soon as possible as the longer you wait, the more suspicious they get and less lenient they will be! Hope this helps and feel free to reach out for any further advice x