You can't specify down to 'file' level, just 'directory' level. So if you want that user only to access specific 'file(s)', then you would put them into a specific directory and lock the user to that directory as 'home', then if you wanted to limit the user to read-only, you would have to chmod the file to readonly for user, group, other.
None of this can be done via the GUI control panel, has to be done by SSH root login to a command shell prompt.