How do bonuses and mortgages work?

Each year I get a performance based bonus which can be quite high, roughly 20% of my salary. However in my contract it explicitly says that its discretionary and the amount one year doesn't correlate with the amount the next years. Does this mean it can't be counted towards a mortgage?

Eg

Year 1: Salary: 50k, bonus 10k Year 2: 50k:5k Year 3: 50k:8k

Would a bank completely ignore the bonus when doing the roughly 4.5x calculation?

I've looked at calculators online and there's no clear answer.