Swap the order of setText
and setHeader
:
mMessage.setText(new String(base64Data));
mMessage.setHeader("MIME-Version", "1.0");
mMessage.setHeader("Content-Type", "application/x-pkcs7-mime; smime-type=signed-data; name=\"smime.p7m\"");
mMessage.setHeader("Content-Transfer-Encoding", "base64");
mMessage.setHeader("Content-Disposition", "attachment; filename=\"smime.p7m\"");