When you get the error: 413 “Request Entity Too Large” error with uploading a file
mkdir /home/dokku/appname/nginx.conf.d/
echo 'client_max_body_size 50M;' > /home/dokku/appname/nginx.conf.d/upload.conf
chown dokku:dokku /home/dokku/appname/nginx.conf.d/upload.conf #I don't know if this is needed
service nginx reload