Board Thread:News and Announcements/@comment-12343352-20190412023225/@comment-4834197-20190412050706

"Open with" here means right-clicking the "hosts" file, then choose "Open with..." on the drop-down menu.

Extensionless means no .txt/.exe/.doc (or the like) at the end of the file name. Extensions are hidden by default, so you can't be sure if it's extensionless, hence enabling the checkbox for "File name extensions" on File Explorer's "View" tab.