I ran into this issue too. It confused me until I realized what was wrong:
You have this:
include /etc/nginx/mime.types;
default_type application/octet-stream;
You want this:
default_type application/octet-stream;
include /etc/nginx/mime.types;
there appears to either be a bug in nginx or a deficiency in the docs (this could be the intended behavior, but it is odd)