Error when uploading file through Drush on S3 volume
It's all there really, it's a security warning and you can fix it by creating a .htaccess file at the root of sites/default/files. The actual content should be:

# Turn off all options we don't need.
Options None
Options +FollowSymLinks

# Set the catch-all handler to prevent scripts from being executed.
SetHandler Drupal_Security_Do_Not_Remove_See_SA_2006_006
&lt;Files *&gt;
  # Override the handler again if we're run later in the evaluation list.
  SetHandler Drupal_Security_Do_Not_Remove_See_SA_2013_003
&lt;/Files&gt;

# If we know how to do it safely, disable the PHP engine entirely.
&lt;IfModule mod_php5.c&gt;
  php_flag engine off
&lt;/IfModule&gt;


Additionally, the .htaccess of the temporary files directory and private files directory (if used) should include this command:

Deny from all


Read more in this security advisory.