(imported comment written by jessewk)
Ben missed a parenthesis in the relevance clause. Try this:
createfile until __END
Windows Registry Editor Version 5.00
{concatenation “%0d%0a” of (("%22%22 %22mtu%22=dword:000004fe") of names of keys (names of adapters whose (exists gateway of it) of network) whose (not exists value “mtu” whose (it != “1278”) of it ) of key “HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces” of registry) }
__END
copy __createfile mtu.reg
waithidden regedit /s mtu.reg