Well, by looking at it, I would venture a guess that it is an 'ideal filter', meaning that it has a binary modulation transfer function (equal to 1 in the bandpass region and 0 outside of it). That is, frequencies in the bandpass range [fbot, ftop] go through the filter with full amplitude, and any unfortunate souls not in that interval get squashed to zero. (Though, I think at a minimum, the zero/baseline frequency and the Nyquist are always filtered out.)
*BUT* I am happy to be corrected by one of the AFNI gurus, if there is something more subtle like a taper I'm neglecting...
--pt
