There is only one file type that uses the .V64 extension:
Nintendo 64 ROM Dump
Data "dumped" to a file from a Nintendo 64 ROM cartridge using Doctor V64; also called a "byteswapped" ROM image; can be played on Mac and Windows computers using N64 emulation software.
Non-byteswapped N64 ROMs typically have a .Z64 extension.
Updated: September 15, 2014