Fix multipart header parsing and add test for attachments with metadata

[wneessen]

Improve handling of multipart headers by addressing edge cases where multiple parameters are present. Add a new test case to validate parsing of EML files with attachments and metadata, fixing GH issue #457.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.37%. Comparing base (208647c) to head (0bb18d6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #458   +/-   ##
=======================================
  Coverage   94.37%   94.37%           
=======================================
  Files          31       31           
  Lines        3555     3558    +3     
=======================================
+ Hits         3355     3358    +3     
  Misses        147      147           
  Partials       53       53           

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