api: expose file.isIncoming as a query param

[atonks2]

The file.isIncoming flag wasn't being honored consistently between files created via JSON or file upload. Additionally, writing an incoming file without SenderSupplied would fail because the writer wasn't checking isIncoming.

This change makes it possible to explicitly specify isIncoming on file creation via query param, and honors the flag in all subsequent requests (get file, get file contents, etc.).

[codecov-commenter]

Codecov Report

Merging #355 (356139d) into master (71d51ac) will decrease coverage by 0.02%.
The diff coverage is 72.72%.

❗ Current head 356139d differs from pull request most recent head 8654fe9. Consider uploading reports for the commit 8654fe9 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #355      +/-   ##
==========================================
- Coverage   74.85%   74.84%   -0.02%     
==========================================
  Files          73       73              
  Lines        8653     8657       +4     
==========================================
+ Hits         6477     6479       +2     
- Misses       1609     1611       +2     
  Partials      567      567