This answer is not correct but it sure does look like it should have the fluorine atoms in a symmetrical arrangement. The problem is that the bromine atom is not sp³d hybridized with a trigonal bipyramidal geometry but rather sp³d² hybridized with an octahedral electron-pair geometry. The bromine atom forms 5 bonds with the fluorine atoms and has one pair of non-bonding electrons left over. This pair of electrons must be in a hybrid orbital and since it is somewhat closer to the nucleus it is place in the square planar part of the octahedral geometry. Thus the fluorine atoms are slightly off to one side resulting in an overall molecular dipole moment.