security (static middleware): fix bowser=true listing all file names from given filesystem root

[aldas]

fix Static middleware listing all files from given filesystem root when browser=true

fixes: #2886

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.06%. Comparing base (0954d6e) to head (c58cc50).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
middleware/static.go	70.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2887      +/-   ##
==========================================
+ Coverage   93.02%   93.06%   +0.04%     
==========================================
  Files          43       43              
  Lines        4456     4458       +2     
==========================================
+ Hits         4145     4149       +4     
+ Misses        194      193       -1     
+ Partials      117      116       -1     

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