I glanced at your mif and it looks ok to me, tomorrow I will do fft in Matlab and send you my results(hopefully).
complex signals in hardware are represented on two (real) channels. One is treated as real and the other as representing the imaginary value(units of sqrt(-1). Don't worry about details now. All you need is that to see amplitude response you look at absolute value i.e. Amplitude of complex vector = sqrt(I^2 + Q^2). plot amplitude and you should see two symmetrical lines unless your input is sine/cosine which leads to one line only(non-symmetrical spectrum). Complex math is same as vector math...