AT-Commands: Send E-mail with attachment
According to the manual, section 5.1.6.10.9. Send mail with attachment - #SMTPCL:


  AT#SMTPCL=&lt;da&gt;,&lt;subj&gt;,&lt;att&gt;[,&lt;filename&gt;,&lt;encod&gt;]
  
  This command permits to send an email with different types of attachments if
  GPRS context has already been activated
  (#SGACT or #GPRS).
  After sending message body text (as with #EMAILD), the command switch to
  online mode if attachment has to be sent.
  While in online mode data received on the serial port are transmitted on the
  SMTP socket as MIME attachment.
  The escape sequence has to be sent to close the SMTP connection


In other words, YOU are responsible for sending the content of the attached file, after you send the body of the email message.