The status 400 is coming from cowboy_rest, when it complains that
handler cowboy_static lacks the functions 'service_available',
'known_methods', 'uri_too_long', and 'allowed_methods'. Which indeed
it does not have.
The only place I mention cowboy_static is in the dispatch rule:
{"/music/[...]", cowboy_static, {dir, "", [
{mimetypes, cow_mimetypes, all}]}},