$ find / -name libffmpeg.so 2> /dev/null
/usr/share/code/libffmpeg.so
/usr/lib64/opera/libffmpeg.so
/usr/lib64/chromium-browser/libffmpeg.so

$ cat /usr/lib64/opera/resources/ffmpeg_preload_config.json 
[
  "lib_extra/libffmpeg.so",
  "/usr/lib/chromium-browser/libffmpeg.so",
  "/usr/lib/chromium-browser/libs/libffmpeg.so"
]

reason is wrong location

Where I should to put it(libffmpeg.so)?

Version:	45.0.2552.812 - Opera is up to date
Update stream:	Stable
System:	Fedora 25 (Twenty Five) (x86_64; KDE)

don't support:
H.264, MP3, AAC
unlike opera on other platform(http://imgur.com/a/roYJD)