Fix access to arbitrary .png files on ruby 1.8
Fix access to arbitrary .png files on ruby 1.8

RosenDimov/sinatra,ktw1222/sinatra,ryanshaw/sinatra,d-bot/sinatra,guidohcosta/sinatra,miyazawatomoka/sinatra,guidohcosta/sinatra,ksw2599/sinatra,mwpastore/sinatra,ksw2599/sinatra,emiliocorpus/sinatra,trexnix/sinatra,kumji/sinatra,porras/sinatra,davydovanton/sinatra,trexnix/sinatra,jkowens/sinatra,nicoayala/sinatra,DoraALin/sinatra,defsprite/sinatra,Wirachmat/sinatra,gferguson-gd/sinatra,lastcat/sinatra,d-bot/sinatra,eileencodes/sinatra,burningTyger/sinatra,mwpastore/sinatra,sinatra/sinatra,wzheng1/sinatra,ABaldwinHunter/sinatra-classic,siberianbear/sinatra,ryanshaw/sinatra,RosenDimov/sinatra,jkowens/sinatra,kumji/sinatra,porras/sinatra,jenalgit/sinatra,midorineko/sinatra,howtolearntocode/sinatra,JonMidhir/sinatra,QLGu/sinatra,sinatra/sinatra,ktw1222/sinatra,burningTyger/sinatra,defsprite/sinatra,jkowens/sinatra,Rayheng/sinatra,ganmacs/sinatra,sethuster/sinatra,autopp/sinatra,DoraALin/sinatra,miyazawatomoka/sinatra,nju520/sinatra,Wirachmat/sinatra,dariubs/sinatra,Rayheng/sinatra,laplaceliu/sinatra,wzheng1/sinatra,dariubs/sinatra,frechei/sinatra,nju520/sinatra,howtolearntocode/sinatra,kyontan/sinatra,siberianbear/sinatra,eileencodes/sinatra,gferguson-gd/sinatra,burningTyger/sinatra,JonRowe/sinatra,lastcat/sinatra,emiliocorpus/sinatra,frechei/sinatra,JonRowe/sinatra,jenalgit/sinatra,davydovanton/sinatra,mwpastore/sinatra,kyontan/sinatra,ABaldwinHunter/sinatra-classic,laplaceliu/sinatra,cyberid41/sinatra,nicoayala/sinatra,sethuster/sinatra,JonMidhir/sinatra,QLGu/sinatra,midorineko/sinatra,ganmacs/sinatra,autopp/sinatra,JonMidhir/sinatra,cyberid41/sinatra