How to access a phone with a broken screen?

My Samsung Galaxy A3 has a broken screen. It still turns on, vibrates when plugged into a charger etc but I need to access my photos on it. I am getting the phone replaced and have been instructed to erase everything that I can off of it, but obviously this is very difficult without access to my phone!

I have plugged it into my iMac, but when I do this my computer does not recognise it. If anyone has a solution to this it would be a great help! Thank you in advance.