Or even the retail brasil 4.4 modem files, it is a newer modem rev then AT&T has and 2600 Mhz LTE (band 7). I have used it and can confirm it works without issue but I didn't use the 2600 Mhz support(but I know 100% it has it)
fastboot flash modem NON-HLOS.bin
fastboot erase modemst1
fastboot erase modemst2
fastboot flash fsg fsg.mbn
Deep in the Heart of the State of the Art with my Moto X.