diff --git a/whisper.sh b/whisper.sh
index 9aa6760..7ccfc68 100755
--- a/whisper.sh
+++ b/whisper.sh
@@ -79,7 +79,7 @@ if [ -z "$FILE" ]; then
 fi
 
 EXTENSION="${FILE##*.}"
-EXTENSION="${EXTENSION,,}"
+EXTENSION=$(echo "${FILE##*.}" | tr '[:upper:]' '[:lower:]')
 VALID_EXTENSIONS="mp3 wav aac ogg flac m4a wma opus aiff alac ape wv amr ac3 dts mka mp4 mkv mov avi webm m4v wmv flv mpg mpeg vob ts mts m2ts 3gp ogv"
 
 if [[ ! " $VALID_EXTENSIONS " =~ " $EXTENSION " ]]; then