Steps to reproduce the issue

Use JMail's addAttachment method's parameter $disposition
For example:

$mail2send = JFactory::getMailer();
$mail->addAttachment($filePath, $fileName, 'base64', 'image/jpeg'. 'inline');

The last parameter is $disposition (here: 'inline').
This parameter never has an effect because JMail does not pass this parameter to the parent class PHPMailer's addAttachment method.

Expected result

The disposition should have an effect.

Actual result

The disposition never has an effect.

System information (as much as possible)

Joomla 3.6.5

Additional comments

None