Downloaded files are always stored as .bin instead of preserving the original extension

Plugin: flutter_cache_manager
Version: ^3.4.1

When downloading media files (e.g., .mp4, .jpg, etc.) using CacheManager.downloadFile(), the cached file is always stored with a .bin extension, regardless of the original URL's file extension.

This creates confusion when trying to:

Use file types based on extension (e.g., previewing .mp4 in a video player)

Debug or manage cache files

Persist/restore with accurate file type outside the cache manager

Start at the CacheManager.downloadFile() entry point and trace how downloaded cache filenames are generated. Verify the behavior with media URLs such as .mp4 and .jpg, then confirm that cached files preserve the original extension instead of always using .bin.

索引モデルが issue の本文から書いたものです。