Use with to open file attachments

[redblacktree]

Ensures that the file is closed even if there are errors in the process. I discovered this issue because I got a non-200 response that caused the client to raise error code 10000, and my subsequent cleanup code failed to delete a temp file because it was already open.

[redblacktree]

I also moved the open() call closer to where it's used

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.72%. Comparing base (73edf4d) to head (4d3b2cc).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #342      +/-   ##
==========================================
+ Coverage   93.45%   93.72%   +0.27%     
==========================================
  Files         103      103              
  Lines        3924     3935      +11     
  Branches      171      174       +3     
==========================================
+ Hits         3667     3688      +21     
+ Misses        217      209       -8     
+ Partials       40       38       -2     

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