Solved
All I need is cast AttachmentPart.getContent()'s result to MimeMultipart
MimeMultipart mp = (MimeMultipart) attachment.getContent();
for (int i = 0; i < mp.getCount(); i++) {
Part bp = mp.getBodyPart(i);
if (bp.isMimeType("text/*")) {
String text = (String)bp.getContent();
//process text
} else if (bp.isMimeType("image/*")) {
InputStream is = bp.getInputStream();
//process image
}
}