See JMailer. In his code, he shows all the headers needed. Possibly you don't give other headers required for a Content-ID to be succesfull.

$encodedImage = '';
# $encodedImage = "--__Middle-Boundary__\n";
$encodedImage .= "Content-Type: image/$type\;  name=\"$cleanName\"\n";
+          # <----- !!!!!!
$encodedImage .= "Content-Transfer-Encoding: base64\n";
$encodedImage .= "Content-Disposition: inline; filename=\"$cleanName\"
+\n";
$encodedImage .= "Content-ID: <img$count>";
$encodedImage .= "\n\n";
$encodedImage .= $self->_encode64(\$inline_image);
[download]