feat: track where pending files are merged into and report Filename on FileUploaded

[adamdecaf]

Fixes: #147

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 74.48980% with 25 lines in your changes are missing coverage. Please review.

Project coverage is 47.70%. Comparing base (550964b) to head (b5494ae).

Files	Patch %	Lines
internal/pipeline/merging.go	75.00%	14 Missing and 3 partials ⚠️
internal/pipeline/aggregate.go	66.66%	6 Missing and 1 partial ⚠️
internal/pipeline/mock_xfer_merging.go	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #226      +/-   ##
==========================================
+ Coverage   47.25%   47.70%   +0.45%     
==========================================
  Files          97       97              
  Lines        4313     4366      +53     
==========================================
+ Hits         2038     2083      +45     
- Misses       1945     1952       +7     
- Partials      330      331       +1     

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