Add option to allow empty attachments

[meain]

Closes #96

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (e85597e) 52.63% compared to head (6628ef0) 52.51%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
attach.go	40.00%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
- Coverage   52.63%   52.51%   -0.13%     
==========================================
  Files           7        7              
  Lines        1290     1293       +3     
==========================================
  Hits          679      679              
- Misses        542      545       +3     
  Partials       69       69              

Flag	Coverage Δ
unittests	52.51% <40.00%> (-0.13%)	⬇️

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