I found some of the company's program is relatively old
mime_mail.php
To Mail a letter

Therefore, pay special attention to when to send attachments

If you are using phpmailer directly attached file name like
Such as

$mail->AddAttachment("images/phpmailer.gif");


But to use when using mime_mail.php

//Read the archives

$fd = fopen("data.jpg", "r");

$data = fread($fd, filesize("data.jpg"));

fclose($fd);

//Objects placed in the mail

$mail->add_attachment($data,data.jpg,"application/octet-stream");


The default format is  application/octet-stream

Official website

There are listed here

gif GIF-images
jpg Jpeg-images
png PNG-images
pdf PDF-socuments
ps Postscript-files
exe Win-/DOS-executable
txt Text-files
html HTML-files
htm HTML-files
mp3 MP3-files
ra RealAudio-files