Wellington (AFP) - Internet mogul Kim Dotcom's legal case returns to court in New Zealand Monday for what may be the Megaupload founder's final chance of avoiding extradition to the United States.

Dotcom's epic extradition battle has dragged on for more than six years, starting in January 2012 when armed police acting on an FBI request raided his Auckland mansion.

The 44-year-old German national is accused of industrial-scale online piracy via his Megaupload empire, which US authorities shut down when the raid took place.

If sent to the United States he will face charges of racketeering, fraud and money laundering carrying jail terms of up to 20 years.

Dotcom and his three co-accused -- Finn Batato, Mathias Ortmann, Bram van der Kolk -- maintain their innocence and will challenge the legitimacy of their arrests this week at the Court of Appeal in Wellington.

If the appeals court rules Dotcom is eligible for extradition, a process that could take months, he will have limited options for overturning the decision.

Theoretically, he could go to the Supreme Court, but his legal team would need compelling new evidence that he was facing a miscarriage of justice.

- Cloud pioneer -

Megaupload was an early example of cloud computing, allowing users to upload large files onto a server so others could easily download them without clogging up their email systems.

At its height in 2011, Megaupload claimed to have 50 million daily users and account for four percent of the world's internet traffic.

The problem, according to an FBI indictment, was that many of the files shared were copyright-protected films and music.

It alleges Megaupload netted more than $175 million in criminal proceeds and cost copyright owners $500 million-plus by offering pirated content.

While such cloud technology is now commonplace, emerging alternatives such as online streaming means the problem of content piracy remains a live one for US authorities.

As such, a high-profile scalp such as Dotcom would still be viewed as a powerful deterrent by the FBI and US Department of Justice.

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Born Kim Schmitz in Kiel, northern Germany in 1974, Dotcom changed his name in 2005, around the same time he established Megaupload.

He used the wealth generated from his website to fund a lavish lifestyle of racing cars and luxury yachts before moving to New Zealand in 2010.

With his penchant for black clothing and Teutonic accent, Dotcom has likened himself to a James Bond villain, arguing that is why he has been pursued so vigorously.

"I'm an easy target, they needed a villain who's rich, flamboyant and over-the-top like me," he said on 2013.

He has remained outspoken throughout his legal battle and last month marked the sixth anniversary of the Auckland raid by announcing a multi-billion dollar damages claim against the New Zealand government.