Hop, je serai moins bête l'an prochain :
PNG is already a compressed data format. Compressing it with GZIP is not likely to decrease the size, and can in fact make it larger.
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http://stackoverflow.com/questions/11289369/why-png-size-doesnt-change-after-using-http-gzip-compressionThe need for X-Accel-Redirect (and it’s sibling X-Sendfile) comes from two distinct requirements
The need to deliver large files.
The need for those files to not be available to the public.
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http://thedataasylum.com/articles/how-rails-nginx-x-accel-redirect-work-together.html