Use ContentID if provided to lookup attachments

[meain]

File struct now has an optional ContentID field which if provided is used to look up inline attachments in the body instead of Name.

Closes #101

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 52.49%. Comparing base (e26e0d1) to head (8e60fa9).

Files	Patch %	Lines
message.go	0.00%	5 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
- Coverage   52.65%   52.49%   -0.17%     
==========================================
  Files           7        7              
  Lines        1297     1301       +4     
==========================================
  Hits          683      683              
- Misses        545      549       +4     
  Partials       69       69              

Flag	Coverage Δ
unittests	52.49% <0.00%> (-0.17%)	⬇️

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