Why do you want to upload a file? Why not just add your solution as the reference solution? Students will not be able to view that.
Answering the question you asked, there is no way to truely hide files you upload from students. You can add files without telling students, but those files will exist in the same folder students are working in. If they explore, they will be able to see all files that are in their current folder.
You could upload pcoded files. This obfuscates the file contents, making it much more difficult for students to see the contents of your files. Of course, it obfuscates it for you as well, so that doesn't really help you preserve a solution file.