Pull over other default Homebrew Nginx config

* set default MIME type to application/octet-stream
* enable sendfile
* bump keepalive timeout to 65 sec

share/powprox/nginx.conf.erb

@@ -2,8 +2,12 @@
 user <%= ENV['USER'] %> staff;
 
 http {
+  default_type application/octet-stream;
   include mime.types;
 
+  sendfile on;
+  keepalive_timeout 65;
+
   # Provide the Pow host root dir as an Nginx variable so per-app nginx
   # configs can dynamically set server root directories relative to it.
   # We could process per-app config as ERB templates instead, but that'd