A multi-part e-mail message can exists out of different parts.

Like:

Array
(
    [0] => 1
    [1] => 1.1
    [2] => 1.1.1
    [3] => 1.1.2
    [4] => 1.2
)

Each part is a section in the mail message. To loop through every part of the message you need to go like this:

$mime_part = mailparse_msg_get_part($this->message, "1.2"); //1.2 is the binary base64 encoded part of the attachement
     
$body_parts = mailparse_msg_get_part_data($mime_part); 

Body part looks like this:

Array
(
    [headers] => Array
        (
            [content-type] => application/x-zip-compressed;  name="3000_000000.txt"
            [content-disposition] => attachment; filename="3000_000000.txt"
            [content-transfer-encoding] => base64
            [x-attachment-id] => b31f1a24358e7e5_0.1
        )

    [starting-pos] => 4513
    [starting-pos-body] => 4736
    [ending-pos] => 4772
    [ending-pos-body] => 4772
    [line-count] => 6
    [body-line-count] => 0
    [charset] => us-ascii
    [transfer-encoding] => base64
    [content-name] => 3000_000000.sbd
    [content-type] => application/x-zip-compressed
    [disposition-filename] => 3000_000000.sbd
    [content-disposition] => attachment
    [content-base] => /
)