Unknown ways to insert inline images in emails?

Recently received a newsletter in gmail and saw the images immediately, without any "Show images" and for some reason I looked at the source. I thought that there normal enclosure with a Content-Disposition: inline and reference it in the body. But saw something different. Pratasenya pictures were designed:

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Content-Type: image/jpeg; name=image.jpg

Content-Transfer-Encoding: base64

Content-Disposition: attachment; filename=image.jpg

Content-Location: example.com/image.jpg

And the message body was just tags example.com/image.jpg">


Tried to reproduce using SwiftMailer'a:

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$transport = Swift_SmtpTransport::newInstance();
$transport->setUsername('login');
$transport->setPassword('password');


$mailer = Swift_Mailer::newInstance($transport);
$message = Swift_Message::newInstance();
$message->setSubject('Hello man!');
$message->setFrom(array('[email protected]'=>'R2D2'));
$message->setTo('[email protected]', 'C3PO');


$body='example.com/image.jpg">';
$message->setBody($body, 'text/html');


$attach=Swift_Attachment::fromPath('/home/example/htdocs/image.jpg');
// to the normal attach add a Content-Location header
$attach->getHeaders()->addTextHeader('Content-Location', 'example.com/image.jpg');


$message->attach($attach);
$mailer->send($message);

It works, images are loaded in a letter once a server is requested as attache not shown. Tested in gmail and outlook 2007. Checked all the rfc I found — never seen mention of such. For rfc to insert inline images you need to use Content-Disposition: inline Content-ID


Question: why does it work?

By the way, gmail always show inline images from?