bugfix - set folder name for File field

[jsirish]

fixes #16

[codecov]

Codecov Report

Merging #17 into master will increase coverage by 0.43%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master      #17      +/-   ##
============================================
+ Coverage     69.01%   69.44%   +0.43%     
  Complexity       21       21              
============================================
  Files             2        2              
  Lines            71       72       +1     
============================================
+ Hits             49       50       +1     
  Misses           22       22

Flag	Coverage Δ	Complexity Δ
#php	69.44% <66.66%> (+0.43%)	21 <0> (ø)	⬇️

Impacted Files	Coverage Δ	Complexity Δ
src/Elements/ElementFileList.php	53.12% <0%> (ø)	8 <0> (ø)	⬇️
src/Model/FileListObject.php	82.5% <100%> (+0.44%)	13 <0> (ø)	⬇️

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