Well, I quickly login into one of my yahoo mail account so as to find a way of rectifying this problem and I discover that I have not yet switched to the new yahoo mail account.
1) Login into your yahoo account the normal way.
2) Save any of this two links or both below in your draft folder. Just save the draft with whatever name you like. See the image below.
3) Right click or press CTRL + Click to go to Yahoo mail Classic whenever you want to send an attachment.
Please, use this method anytime you want to switch-over to Yahoo mail Classic. So, save those links in a in a secured place. Am still checking for a permanent method of switching over to Yahoo mail classic.