You appear to be missing some stuff off the end of the 'disposition => " ... "' line, is this an artifact of cut & paste, or a possible cause of the error?

$sender->SendFile ( {
                description => 'Company logo',
                ctype       => 'image/gif',
                encoding    => 'base64',
                disposition => "inline; filename=\"picture.gif\";\r\nC
+ontent-ID: <logo>",
                file        => 'picture.gif'} );
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