Fix webm extension mistakingly guessed as gif (fix #438)
Fix webm extension mistakingly guessed as gif (fix #438)

YoukaiCat/imgbrd-grabber,YoukaiCat/imgbrd-grabber,Bionus/imgbrd-grabber,Bionus/imgbrd-grabber,Bionus/imgbrd-grabber,Bionus/imgbrd-grabber,Bionus/imgbrd-grabber,Bionus/imgbrd-grabber