Fix attachment merging

[d-buchmann]

Allow auto-merging attachments with the same external path (if they have one), disregarding the internal path.

Until now, if there is a part with attachments from an info provider and you chose to download them, when you update the part from the info provider these attachmets get duplicated and you can't save unless you delete the duplicates by hand.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.76%. Comparing base (1521c63) to head (47431d7).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1498      +/-   ##
============================================
- Coverage     59.77%   59.76%   -0.01%     
- Complexity     9103     9104       +1     
============================================
  Files           679      679              
  Lines         29618    29621       +3     
============================================
- Hits          17703    17702       -1     
- Misses        11915    11919       +4     

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[jbtronics]

Seems useful.
Thanks.