There could be no or more than one file attachments present with an outgoing message.
File data and file name is present with an attachment. File data may be extracted from some source like data store or application data file. Attachment’s MIME type is determined by the filename.
Below is given MIME types list and also their file name extensions which are permitted for the attachments, to an email. You are not just bound to such extensions. If an unknown extension is used,