Redirects your local environment's Drupal install /sites/*/files/* requests to a server that actually has them. This helps avoid having to sync (sometimes, very large) file structures onto your local machine and taking up valuable hard-drive space. Source: http://dropbucket.org/node/337

drupal_redirect_for_files_dir.conf

### Apache Rewrite
<IfModule mod_rewrite.c>
  RewriteEngine on
  # Force image styles that have local files that exist to be generated.
  RewriteCond %{REQUEST_URI} ^/sites/([^\/]*)/files/styles/[^\/]*/public/((.*))$
  RewriteCond %{DOCUMENT_ROOT}/sites/%1/files/%2 -f
  RewriteRule ^(.*)$ $1 [QSA,L]
  # Otherwise, send anything else that's in the files directory to the
  # production server.
  RewriteCond %{REQUEST_URI} ^/sites/[^\/]*/files/.*$
  RewriteCond %{REQUEST_URI} !^/sites/[^\/]*/files/css/.*$
  RewriteCond %{REQUEST_URI} !^/sites/[^\/]*/files/js/.*$
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  RewriteRule ^(.*)$ http://dev.example.com/$1 [QSA,L]
</IfModule>

### nginx Rewrite
## Try local files first, otherwise redirect to a development server.
location ^~ /sites/default/files {
  try_files $uri @dev_redirect;
}
## Development Server Redirect
location @dev_redirect {
  rewrite ^ http://dev.example.com$request_uri? permanent;
}