I found this reference, but I don't know how to implement the content_type header: https://www.mathworks.com/matlabcentral/fileexchange/18351-send-mail/content/sendmail%20with%20jgmail%20class%20java%20package/docs_jgmail/com/messners/mail/MessagePart.html There's something like:
public MessagePart(java.lang.String name,
java.lang.String filename,
java.lang.String content_type,
java.lang.String encoding)
