[Mac] Server won't set secure_file_priv from /etc/my.cnf

[Bruce Preston]

I've just installed MySQL Community Server 5.7.16 on OS X El Capitan.

My configuration file (/etc/my.cnf) looks like this:

[mysqld]
datadir = /usr/local/mysql/data
secure_file_priv = /Users

and if I run

mysqld --verbose --help

or Workbench, it says the system variable is set.

However if I execute

SELECT @@secure_file_priv;

it comes back as NULL, any attempt to LOAD DATA INFILE comes back with Error 1290 and the log file explicitly states:

2016-11-01 19:59:01.370938, 0, Note, --secure-file-priv is set to NULL. Operations related to importing and exporting data are disabled

The connection is definitely getting its config from /etc/my.cnf (it's listed as such on the information panel on the MySQL Connections screen when opening Workbench). How do I get it to set the system variable appropriately?