solved
location ~.*\.(3gp|gif|jpg|jpeg|png|ico|wmv|avi|asf|asx|mpg|mpeg|mp4|pls|mp3|mid|wav|swf|flv|txt|js|css|html|htm|exe|zip|tar|rar|gz|tgz|bz2|uha|7z|doc|docx|xls|xlsx|pdf|iso)$ {
expires 7d;
try_files $uri @backend;
}
location ~.*\.(html|htm)$ {
add_header Pragma "no-cache";
try_files $uri @backend;
}