Bug fix for converter registration

[tclose]

Fixes issue where the same file can be imported from different file paths (i.e. by pytest)

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 92.63%. Comparing base (1e75998) to head (22246a3).

Files	Patch %	Lines
fileformats/core/mixin.py	60.00%	1 Missing and 1 partial ⚠️
fileformats/core/utils.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #66      +/-   ##
==========================================
- Coverage   92.66%   92.63%   -0.04%     
==========================================
  Files          45       45              
  Lines        4911     4915       +4     
  Branches      547      547              
==========================================
+ Hits         4551     4553       +2     
- Misses        270      272       +2     
  Partials       90       90              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.