Recently a customer found that it's impossible to upgrade 1.3.x installations to 1.4.2 anymore.

The cause is that:

• The old image upgrade script from 1.3.8 checks if an image is an ISO file by grepping it for ISO9660, instead of using file/libmagic or trying to mount it (https://github.com/vyos/vyatta-cfg-system/blob/equuleus/scripts/install/install-image#L179-L181).
• The image for 1.4.1 still had an ISO9660 string in it.
• The ISO image of 1.4.2 and later no longer includes that line, even though it's certainly a valid ISO image file — I'm not sure where the string actually came from, since it's not a part of the ISO9660 standard.
• Due to that misguided check, 1.3.8 considers a perfectly good ISO image invalid.

If we want to make direct upgrade from 1.3.x to anything newer than 1.4.1, we need to artificially include that string in the image to satisfy the strange check.