A .264 file stores video data encoded by the H.264/MPEG-4 AVC codec, it is often recorded by various surveillance digital cameras. Typically however, the video is present in some container and uses their default extensions, such as MKV, MP4, M4A, FLV etc.
Several multimedia converters should be able to handle various .264 formats and convert them to other multimedia files, including Adobe Flash (.flv) files and thus allow you to perform 264 to flv conversion.