I tried many things I found googling but nothing work, until I saw this:

Assuming you are using apache, what you must do is go to you public_html folder, create a file .htaccess and paste in it:

 

 
<FilesMatch "\.(ico|pdf|flv|jpe?g|png|gif|js|css|swf)$">
    ExpiresActive On
    ExpiresDefault "access plus 1 year"
</FilesMatch>

you can add more files in the list.

Thanks for visiting.