Use storage abstraction methods to attach attachment to message.

[sipa-echo-zaoa]

Fix for #413

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.60%. Comparing base (d637505) to head (dbc721e).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #414      +/-   ##
==========================================
+ Coverage   87.58%   87.60%   +0.01%     
==========================================
  Files          17       17              
  Lines        1394     1396       +2     
  Branches      150      150              
==========================================
+ Hits         1221     1223       +2     
  Misses        128      128              
  Partials       45       45              

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