The file name field allows you to enter your own page name and extension.
By default the page name is the same as the Artboard name and the default extension is "html".
For example, an artboard named, "Column Layout" would be exported as "Column_Layout.html".
You can omit the extension or define your own.
For example, you can enter, "Columns", "Columns.php", "Columns.abc" or whatever you choose.
Do not use artboards with the same name if you plan to export them as a single page application or to the same directory as they will overwrite the last one or cause issues in a single page application.