When I do that with my Nexus 8 tablet, all I can do is choose between "Connect as camera", which I suppose means MTP, and "Charge".
Hmm... Nexus 8 is obviously have come from some alternate reality (or perhaps from the future?) thus we can not be 100% sure it behaves like other Nexus devices. This being said with most devices you can only root/reflash/etc if you boot it in special "recovery mode" (and this is good: device which can be easily overwritten by any malicious changer will not be very practical).
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