Retries seemed to be causing zero byte files

[mjh1]

Need to save the contents of the input so that retries won't end up reading from an empty reader.

[codecov]

Codecov Report

Merging #40 (6879e76) into main (7d4f20b) will decrease coverage by 0.60108%.
The diff coverage is 0.00000%.

@@                 Coverage Diff                 @@
##                main         #40         +/-   ##
===================================================
- Coverage   16.12903%   15.52795%   -0.60108%     
===================================================
  Files              2           2                 
  Lines            155         161          +6     
===================================================
  Hits              25          25                 
- Misses           121         127          +6     
  Partials           9           9                 

Files	Coverage Δ
core/uploader.go	20.00000% <0.00000%> (-1.00840%)	⬇️

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Files	Coverage Δ
core/uploader.go	20.00000% <0.00000%> (-1.00840%)	⬇️