Webform file attachment
Out of the box Drupal 8 doesn't support file attachments. Take a look at this module and read the steps on the page: https://www.drupal.org/project/swiftmailer`


  The Swift Mailer module extends the basic e-mail sending functionality
  provided by Drupal by delegating all e-mail handling to the Swift
  Mailer library. This enables your site to take advantage of the many
  features which the Swift Mailer library provides, such as :
  
  
  Sending e-mails directly through a SMTP server of your choice, a locally installed MTA agent such as sendmail or the mail functionality
  provided by PHP.
  Sending HTML (MIME) e-mails.
  Adding file attachments to e-mails.
  Adding inline images to e-mails.
  


Also take a look at this: https://www.drupal.org/node/2821590