fix: remove extra flash import

[eschutho]

SUMMARY

@mistercrunch found this bug:

This removes the extra error message, and also adds an accept on the file input to only allow the user to select the formats that are valid.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

no visual tests available, but all other regression tests should pass

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-commenter]

Codecov Report

Merging #11121 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11121      +/-   ##
==========================================
- Coverage   61.70%   61.64%   -0.07%     
==========================================
  Files         816      826      +10     
  Lines       38586    38973     +387     
  Branches     3650     3669      +19     
==========================================
+ Hits        23808    24023     +215     
- Misses      14598    14769     +171     
- Partials      180      181       +1     

Flag	Coverage Δ
#javascript	62.20% <ø> (+0.02%)	⬆️
#python	61.30% <ø> (-0.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../src/views/CRUD/data/savedquery/SavedQueryList.tsx	64.10% <0.00%> (-35.90%)	⬇️
superset/utils/decorators.py	50.76% <0.00%> (-4.24%)	⬇️
...rontend/src/SqlLab/components/TabbedSqlEditors.jsx	72.72% <0.00%> (-0.96%)	⬇️
...erset-frontend/src/datasource/DatasourceEditor.jsx	63.63% <0.00%> (-0.59%)	⬇️
superset/db_engine_specs/presto.py	81.42% <0.00%> (-0.54%)	⬇️
superset/utils/pandas_postprocessing.py	76.47% <0.00%> (-0.50%)	⬇️
superset/views/utils.py	84.21% <0.00%> (-0.10%)	⬇️
superset/config.py	90.07% <0.00%> (-0.04%)	⬇️
superset/errors.py	100.00% <0.00%> (ø)
superset/charts/api.py	74.56% <0.00%> (ø)
... and 26 more

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