MLPAB-1685: Show relevant error when not selecting a file to upload

[acsauk]

Purpose

Fixes MLPAB-1685

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (01d59d5) 91.94% compared to head (b0690f6) 91.95%.
Report is 1 commits behind head on main.

❗ Current head b0690f6 differs from pull request most recent head e244af4. Consider uploading reports for the commit e244af4 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #985   +/-   ##
=======================================
  Coverage   91.94%   91.95%           
=======================================
  Files         180      180           
  Lines        9848     9854    +6     
=======================================
+ Hits         9055     9061    +6     
  Misses        677      677           
  Partials      116      116           

Flag	Coverage Δ
unittests	91.95% <100.00%> (+<0.01%)	⬆️

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