in my case, I can log the $request_body in the access_log via proxy_pass, even when uploading files more then 1MB.

I just wanted to limit the size of the $request_body in the log. Here is my nginx.conf:

http {
include mime.types;
default_type application/octet-stream;

map $request_body $request_body_short {
"~^(?<SHORT>.*filename.*)Content-Type" $SHORT;
default $request_body;
}

log_format main '$remote_addr - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" "$http_user_agent" "$http_x_forwarded_for" $request_time '
'"$request_body_short" $http_uid "$http_build" "$http_appversion" "$uid_got" "$host"';

access_log logs/access.log main;

....
}

if I changed the map default value to something like 'wrong', the logged $request_body would be 'wrong'.