nginx adds a `content-type: application/octet-stream` header by
default, and `add_header` does not supplant it. By setting
`default_type` instead we avoid the extraneous header.
I'm using this config on my homeserver and while trying out alternative
Matrix clients I discovered (pun intended) that the auto-discovery of my
homeserver is broken. While investigating I found out that neither the
JS nor the Rust SDK (tested via element-web and fractal) are happy about
an empty `m.identity_server`-block. Removing this part fixed the problem
for me.
