Fix issue reusing FileDownload model

[philippjfr]

Fixes #4321

[codecov]

Codecov Report

Merging #4328 (8257f96) into main (58adf02) will decrease coverage by 12.41%.
The diff coverage is 35.00%.

@@             Coverage Diff             @@
##             main    #4328       +/-   ##
===========================================
- Coverage   82.67%   70.26%   -12.41%     
===========================================
  Files         239      240        +1     
  Lines       34514    34554       +40     
===========================================
- Hits        28533    24279     -4254     
- Misses       5981    10275     +4294     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	70.26% <35.00%> (-3.60%)	⬇️

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Impacted Files	Coverage Δ
panel/widgets/misc.py	94.44% <ø> (ø)
panel/tests/ui/widgets/test_misc.py	33.33% <33.33%> (ø)
panel/models/widgets.py	100.00% <100.00%> (ø)
panel/pane/vtk/synchronizable_deserializer.py	0.00% <0.00%> (-97.59%)	⬇️
panel/tests/ui/widgets/test_tabulator.py	9.59% <0.00%> (-89.40%)	⬇️
panel/tests/ui/widgets/test_input.py	11.34% <0.00%> (-88.13%)	⬇️
panel/tests/ui/layout/test_accordion.py	14.72% <0.00%> (-85.28%)	⬇️
panel/tests/ui/widgets/test_sliders.py	15.42% <0.00%> (-84.58%)	⬇️
panel/tests/ui/layout/test_card.py	18.01% <0.00%> (-81.99%)	⬇️
panel/tests/io/test_notebook.py	20.83% <0.00%> (-79.17%)	⬇️
... and 54 more

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