FIX: Qwarp in AFNI can have NIFTI and NIFTI_GZ as extensions

[oesteban]

Replaces #2921 from @gpiantoni (apologies for the force push).

[codecov]

Codecov Report

Merging #3028 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #3028      +/-   ##
=========================================
- Coverage   67.51%   67.5%   -0.01%     
=========================================
  Files         344     344              
  Lines       44058   44062       +4     
  Branches     5552    5553       +1     
=========================================
  Hits        29746   29746              
- Misses      13566   13570       +4     
  Partials      746     746

Flag	Coverage Δ
#smoketests	50.29% <ø> (ø)	⬆️
#unittests	64.96% <0%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/afni/preprocess.py	81.97% <0%> (-0.38%)	⬇️

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