If I email an image from Irfanview (using an SMTP connection), it has a MIME type of application/octet-stream - this means that my email reader (Thunderbird) does not know the file type, and does not display the image. The correct MIME type for a JPEG file should be image/jpeg, and when I send JPEGs as an attachment from my mail reader, it correctly sends as image/jpeg, and correctly displays the image when it is received.