Fix logic to define the image extension

build.sh

@@ -138,7 +138,9 @@ for KALI_ARCH in $KALI_ARCHES; do
 	fi
 	set -e
 	IMAGE_EXT="${IMAGE_NAME##*.}"
-	IMAGE_EXT="${IMAGE_EXT:-img}"
+	if [ "$IMAGE_EXT" = "$IMAGE_NAME" ]; then
+		IMAGE_EXT="img"
+	fi
 	mv -f $IMAGE_NAME $TARGET_DIR/kali-linux-$KALI_VERSION-$KALI_ARCH.$IMAGE_EXT
 	mv -f build.log $TARGET_DIR/kali-linux-$KALI_VERSION-$KALI_ARCH.log
 done