Just an FYI, the WordPress 5.0.1/4.9.9 (and others back to 3.7.x) security update broke uploads for CSV files under certain circumstances.

The trac ticket is here and the workaround is here.

This note says that it will be addressed in 5.0.3 which will probably correlate to 4.9.10 (unless 5.0.2 also includes security updates).