[Q] No download modus

Hey guys!

i have (had) a rooted S3 GT-I9300. I wanted to unroot and wipe it. Because i did not read carfully enough i uninstalled the SuperSU-app so i could not boot into download mode anymore. Everything i can boot in the the cwm-based Recovery v5.5.0.4 mode.
I tried to load a new firmware from an SD-Card, but it always tells me "Can't open /sdcard/I9300XXELLA_I9300OXAELLA_BTU.zip".

Any suggestions?