FIX: Incorrect extension identified when checking File traits

[oesteban]

Rewritten the validation of filename extensions, as it was incorrectly
interpreting the extension.

Fixes #2984.

[codecov]

Codecov Report

Merging #2987 into master will decrease coverage by 3.35%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2987      +/-   ##
==========================================
- Coverage    67.6%   64.24%   -3.36%     
==========================================
  Files         344      342       -2     
  Lines       43802    43747      -55     
  Branches     5478     5475       -3     
==========================================
- Hits        29611    28106    -1505     
- Misses      13475    14553    +1078     
- Partials      716     1088     +372

Flag	Coverage Δ
#smoketests	?
#unittests	64.24% <100%> (-0.8%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/base/traits_extension.py	89.52% <100%> (-5.24%)	⬇️
nipype/interfaces/nilearn.py	40% <0%> (-56.67%)	⬇️
nipype/utils/spm_docs.py	25.92% <0%> (-44.45%)	⬇️
nipype/interfaces/freesurfer/base.py	50% <0%> (-30.51%)	⬇️
nipype/utils/logger.py	59.7% <0%> (-29.86%)	⬇️
nipype/algorithms/rapidart.py	35% <0%> (-29.42%)	⬇️
nipype/interfaces/spm/base.py	57.94% <0%> (-29.14%)	⬇️
nipype/utils/provenance.py	55.73% <0%> (-28.35%)	⬇️
nipype/interfaces/fsl/model.py	55.26% <0%> (-25.35%)	⬇️
nipype/testing/fixtures.py	77.33% <0%> (-21.34%)	⬇️
... and 41 more

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