ffmpeg how to allow a file extension

It's a private option of the HLS demuxer, so

ffmpeg -allowed_extensions ALL -i "C:\streamingtest.m3u8" -c copy "test.ts"

I think this is directive for the player -allowed_extensions

try the following:

ffplay -allowed_extensions ALL index.m3u8

it is working form me with the key stored in the local folder