As shown in the attached image, you can easily find the URL of any uploaded file in Moodle 2.4+ for any use.
For example, when the URL of a file is known using Repository File Manager Plug In, similar to Moodle 1.9 file system, you may link Excel files to questions in a test.
Concerning the place where to load the files linked to questions, you may use the normal File resource where you upload, second to a first file the student can see, a second, or more, file the students can't see.
All these files have the same URL except the name of the file at the end of URL.
Students have the right to see these files because the resource is not hidden.
This is not true when you create a directory, upload the files to this directory, and hide the directory after so the students don't see the files used in tests: student have no access to the files because these files are hidden.