Fix BIDSelect confusion between Nifti and JSON

[adelavega]

Fixes #192

[codecov-io]

Codecov Report

Merging #193 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #193   +/-   ##
======================================
  Coverage    76.4%   76.4%           
======================================
  Files          18      18           
  Lines        1017    1017           
  Branches      177     177           
======================================
  Hits          777     777           
  Misses        150     150           
  Partials       90      90

Flag	Coverage Δ
#ds003	76.4% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
fitlins/workflows/base.py	60.43% <ø> (ø)	⬆️

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[effigies]

Should probably allow .nii as well.

[effigies]

What about .gii?

[adelavega]

Sure, why not? Too bad we can't say "not JSON"