WeCom's CDN returns `content-type: application/octet-stream` for images.
`mimetypes.guess_extension` maps that to ".bin", a truthy value that
short-circuited `_guess_extension()` before the magic-byte detector ran, so
every inbound image was cached as `img_*.bin`.
Treat ".bin" as "unknown" so the URL suffix / magic-byte fallback decides.
The second half of the report (URL-encoded, unpadded `aeskey`) is already
handled on main by `_decrypt_file_bytes`.
Salvage of PR #10187 by @nightq onto `plugins/platforms/wecom/media.py`.
Fixes#10085
f39680de45