When sending a Mass Email, Attachments can be added in step 2: Options. Under the Attachments section:
  1. Click Browse to locate the file.
  2. Click Upload to add it to the email. (Note: As files are added, note the file size limits below the listed files as these cannot be exceeded.)
  3. Repeat steps 1 and 2 for any additional file(s).
  4. Once added, click Next in the upper left to continue with the Mass Email setup.