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Bloksel websites cannot log MAC they log the IP, it has to do with how TCP/IP protocol works, MAC is layer2 and will keep changing all the way, the packet arriving to the webserver has the MAC of previous hop which is most likely the router or load balancer before it. That's why he has the code to tag the file with MAC instead of just getting it from the communication path during upload.
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Karnagious, MAC disclosing or logging is not a security risk, it is a privacy risk, meaning that it cannot be used to hack but can be used to identify people by associating IP and MAC, many people were identified and arrested through their MAC addresses, the link you posted even lists one story.
Edit: To clarify even more, MAC can be very useful once you are inside a compromised network and you want a specific machine, as MAC doesn't change (unless a user spoofs a MAC) it is the best way to identify a certain machine, also can be used for packet capturing between the machine and router/gateway. So I wouldn't say it is not a security risk.