'Choose file' sidebar options disappeared
The kind of items in your case hidden are named Favourites. To show them, go to a site where you can upload a file (e.g. https://encodable.com/uploaddemo/) and then click on "Choose File", like you said. When you are prompted to upload something, just hover your cursor on the Favourites line, located on the top of the sidebar. A "Show" button will be shown (no irony, at all). Click it and all of the items will re-appear. Done!

If you have any more issues with this, feel free to comment.