Shimmie2 on nginx (Rewrite)

Question

Hello guys, I'm porting the rewrite rules of shimmie2 to nginx and ran across a problem: From the first impression it worked quite well, but when you try to view the image (I mean via right-click menu in your browser) you end up with stuff like this, instead the original image:

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These are the orginal rewrite rules:

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ index.php?q=$1&%{QUERY_STRING} [L]
RewriteRule ^_images/([0-9a-f]{2})([0-9a-f]{30}).*$  images/$1/$1$2 [L]
RewriteRule ^_thumbs/([0-9a-f]{2})([0-9a-f]{30}).*$  thumbs/$1/$1$2 [L]

and these the ones I created:

rewrite "^/_thumbs/([0-9a-f]{2})([0-9a-f]{30}).*$" /thumbs/$1/$1$2 last;

rewrite "^/_images/([0-9a-f]{2})([0-9a-f]{30}).*$" /images/$1/$1$2 last;

if (!-e $request_filename) { rewrite ^/(.*)$ /index.php?q=$1&$query_string last; }

Where is my fault? Thank you in advance