I can not upload CSV files to WordPress

This is a WordPress bug introduced with version 4.7.1 and will be fixed with 4.7.3.
Please look at the info on below pages:

1. https://wordpress.org/support/topic/error-message-cant-upload-any-csv-file-in-media-library/
2. https://wordpress.org/support/topic/read-this-first-wordpress-4-7-master-list/?view=all#post-8521428

The suggestion is to use Disable Real MIME Check plugin and remove it as soon as 4.7.3 is released.