We currently have 1 dataset that provide a mask (Haxby) and the mask is a nifti file with float values (0., 1.). If I remeber well, FSL and SPM also output float files when you compute a mask.
So if we forbid any other type than int, that will force people to load and convert manually the masks intead of giving the path directly to the Nifti Masker.

Maybe we could check that the image has only 2 values as it is done in intersect masks ?
    assert len(np.unique(mask)) == 2, "Meaningful error message"