Consolidate filename normalization for exports

[rosteen]

This consolidates the filename logic for exporting to fix the standalone case for all export types, not just movies. I wasn't sure whether to put the changelog in with the other Export stuff or in Bug Fixes, since I think the standalone issue was present in 3.8.

[pllim]

Looks like one test case broke:

cubeviz/plugins/tests/test_export_plots.py::test_export_movie_cubeviz_exceptions - AttributeError: 'str' object has no attribute 'parent'

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.82%. Comparing base (f1ba4a6) to head (09f4b85).
Report is 16 commits behind head on main.

❗ Current head 09f4b85 differs from pull request most recent head 5f5d88a. Consider uploading reports for the commit 5f5d88a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2782      +/-   ##
==========================================
+ Coverage   88.72%   88.82%   +0.09%     
==========================================
  Files         110      110              
  Lines       16353    16425      +72     
==========================================
+ Hits        14509    14589      +80     
+ Misses       1844     1836       -8     

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[rosteen]

@pllim I fixed that test, the remaining failure is a failed remote data download, not anything to do with this PR.

[kecnry]

This makes sense to me and also am glad to see the filetype handling reusable rather than copy-pasted everywhere. Thanks!

[pllim]

Nice clean up. Thanks!