Let’s talk about files. They come in different lengths, and different coarseness of cut. They range from bastard (coarsest) through second cut, to dead smooth (finest). My first cuts were made with a 12-inch bastard and were finished with a 10 inch dead smooth file. Remember that files only cut on the forward stroke. Do not allow the file to drag over the work surface during the backstroke or you will dull the teeth. Files should be cleaned with a special wire brush called a file card. A clean file allows its teeth to cut to their full depth, and for the chips to fall away while cutting. Best results will be achieved when your project is clamped in a vice. This minimizes "chatter" and will allow you to use both hands when using the file. While you have the file in your hand, lightly file all of the rough edges on the bracket, or anything else that might snag your umbrella’s fabric cover. Incidentally, I'm not particularly proud of my file work, but in this case, good enough is.
A final note on clamping cold shoes. I prefer the Stroboframe Shoe for the better build quality. I must admit that cheaper "Made In China" alternatives are out there, and I'm sure they will suffice. I have had some of Chinese versions strip their threads. You get what you pay for, I guess.