fix wrong IndexFile truncate

[alessio-perugini]

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What kind of change does this PR introduce?

Fixes a regression that truncates every . in the filename.

What is the current behavior?

What is the new behavior?

Does this PR introduce a breaking change, and is titled accordingly?

Other information

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (6aa1be0) 63.04% compared to head (7ad3dab) 63.10%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2346      +/-   ##
==========================================
+ Coverage   63.04%   63.10%   +0.06%     
==========================================
  Files         201      201              
  Lines       19259    19263       +4     
==========================================
+ Hits        12141    12156      +15     
+ Misses       6064     6056       -8     
+ Partials     1054     1051       -3     

Flag	Coverage Δ
unit	63.10% <87.50%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
arduino/resources/index.go	46.55% <87.50%> (+1.01%)	⬆️

... and 1 file with indirect coverage changes

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[per1234]

I verified this fixes #2345.

Thanks Alessio!