Webform email twig template questions
That is a great question. 

Getting just the filename is not currently supported but you can pull the file's URL using [webform_submission:values:join_a_file:delta:format:url]

The current list of supported formats for uploaded files is here. I created a ticket to 
add machine name to element submission display formats so that this information is exposed to site builder.

If you need just the file name, please create a new ticket in the Webform module's issue queue.