The concept is:
1. Create an array with your file attachment data:
Code:
$att_array = array('file' => $attachment_file, 'name' => basename($attachment_file), 'mime_type'=>$attachment_filetype);
Of course, substituting the correct information for each of those $attachment_xxxxxx variables:
$attachment_file is the actual complete physical path (on the server) to the file being attached -- that is, wherever you moved the files to during your upload
$attachment_filetype is the MIME type for that file. If you skip the 'mime_type' part of this array, it will use application/octet-stream as a safe fallback.
If you need to handle submitting multiple files, you can make it a nested array.
2. Then include that array in your email data:
Code:
zen_mail(STORE_NAME, SEND_TO_ADDRESS, EMAIL_SUBJECT, $email_text, $first_name . ' ' . $last_name, $email_address, array('EMAIL_MESSAGE_HTML' => $email_html), 'band_signup', $att_array);
3. MIME-HTML must be enabled in your Admin->Configuration->Email Options settings.