.FFINDEX File Extension
What is an FFINDEX file?
Index of keyframe/sample positions created by FFmpegSource2, a wrapper library for FFmpeg; contains data used for seeking media during playback; created when audio or video files are opened if an index does not already exist; may also be created manually with the FFindex function.
FFmpegSource2 creates FFINDEX files by default when media files are indexed. You can disable the default behavior if you don't want these files to be created.
FFmpegSource2 is also available as a plugin for the program Avisynth and creates FFINDEX files in the same manner.
Common FFINDEX Filenames
[Source File Name].ffindex - Default name FFmpegSource2 gives to index files.