fix bug in handling of multi-level multi-file images (e.g. image-[]-[].mif)
fix bug in handling of multi-level multi-file images (e.g. image-[]-[].mif)

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3