Below is the largest bag I use for work. It’s wide enough to hold files. It is also structured and have a zipped closure. The problem is, the strap is most likely not long enough for your needs. They can be worn on the shoulder but it sounds like you want a longer strap. Also, I think it can get heavy if too much is placed in the bag. It’s midnight navy so it can take inclement weather. I used it in rain, wind and snow.
The Givenchy Antigona looks like the perfect bag to meet all your requirements. The strap is a nice length too.

The Givenchy Antigona looks like the perfect bag to meet all your requirements. The strap is a nice length too.
