Rather trivial.  It takes NO particular skill, no memory effort, except for this tip this is, and no post-it . . .   It only takes discipline.  You shall make the oath that from now on, as soon as you finish typing“attached”, as in“please find attached”, or "you’ll find attached", as soon as your fingers release the d key, you will stop typing, you will grab your mouse, and you will fetch and attach the file or files you just quoted.  This is it!

What if there are many files to be attached?

Well, you still lift the fingers off the keyboard as soon as the d key goes up, and you recall what was supposed to be attached, what was the next word going to be? Please find attached the singular object, or the plural object.  If plural, how many? Are they located in various places? For each object, go and grab it with the mouse to attach it to your email.
Once this is done, go back to your keyboard, and continue typing the missing words, the ones you’ve just been enumerating mentally.

I have kept this article concise on purpose, as there should be as little to remember for the method to be memorised, and readily availabel next time the opportunity arises.  I hope it will have helped.