A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Ok, FILES is indeed working, and working properly in Excel 2016. The problem is the file nomenclature that is needed fro Excel 2016. The Apple HFS (using colons as node separators) file system in effect up through Office 2011 has been deprecated by Apple and is no longer supported in Office 2016. You must use the POSIX file notation when specifying files in Office 2016, which you are doing.
There is a bug in Excel 2016 where Sandboxing prevents FILES from returning the directory contents. What should happen is that the OS should present a user dialog asking permission to access that directory. A bug is filed within Microsoft about this issue. It remains to be seen when, or if, it will be addressed.