Add file, url and fields methods to backends.
Add file, url and fields methods to backends.

fxtentacle/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,tizzybec/tilemill,florianf/tileoven,mbrukman/tilemill,tilemill-project/tilemill,paulovieira/tilemill-clima,tizzybec/tilemill,tilemill-project/tilemill,MappingKat/tilemill,MappingKat/tilemill,paulovieira/tilemill-clima,tilemill-project/tilemill,nyimbi/tilemill,makinacorpus/tilemill,tizzybec/tilemill,fxtentacle/tilemill,mbrukman/tilemill,nyimbi/tilemill,isaacs/tilemill,nyimbi/tilemill,florianf/tileoven,nyimbi/tilemill,florianf/tileoven,isaacs/tilemill,MappingKat/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,nyimbi/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,mbrukman/tilemill,fxtentacle/tilemill,Zhao-Qi/tilemill,fxtentacle/tilemill,Zhao-Qi/tilemill,fxtentacle/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,makinacorpus/tilemill,paulovieira/tilemill-clima,mbrukman/tilemill,Zhao-Qi/tilemill,isaacs/tilemill,tizzybec/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,Zhao-Qi/tilemill,Zhao-Qi/tilemill,tizzybec/tilemill,paulovieira/tilemill-clima,MappingKat/tilemill,MappingKat/tilemill,mbrukman/tilemill,paulovieira/tilemill-clima