Fix broken external_file paths when organizing on Windows

[h-mayorquin]

When dandi organize --update-external-file-paths runs on Windows, the external_file paths written into NWB files use backslashes instead of forward slashes.

Expected:

ses-2026-02-12-1_image/21b52ce3_external_file_0.mp4

On Windows before this fix:

ses-2026-02-12-1_image\21b52ce3_external_file_0.mp4

Downstream tools expecting forward slashes (as DANDI/S3 asset keys always use) fail silently when resolving these paths against the archive. This was observed in Dandiset 001771 and reported in catalystneuro/nwb-video-widgets#33, where get_dandi_video_info() returned empty results. A defensive workaround was merged in catalystneuro/nwb-video-widgets#38, but the root cause is here in dandi organize.

The fix has two parts:

• Write side (organize.py): _create_external_file_names() used os.path.join to build external file paths. On Windows that produces backslashes. Replaced with posixpath.join since these are DANDI/S3 keys, not local filesystem paths.
• Read side (pynwb_utils.py): _get_image_series() wrapped external file strings in Path(), which on Windows converts forward slashes back to backslashes. Replaced with PurePosixPath.

I have also added a regression assertion in test_video_organize that external_file values never contain backslashes. The initial push included only the write-side fix; Windows CI caught that Path() on the read side was re-introducing backslashes, confirming the assertion works as intended.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.27%. Comparing base (fa33e8d) to head (b885ad6).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
dandi/organize.py	50.00%	1 Missing ⚠️
dandi/pynwb_utils.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1832      +/-   ##
==========================================
- Coverage   76.28%   76.27%   -0.02%     
==========================================
  Files          87       87              
  Lines       12482    12484       +2     
==========================================
  Hits         9522     9522              
- Misses       2960     2962       +2     

Flag	Coverage Δ
unittests	76.27% <66.66%> (-0.02%)	⬇️

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[h-mayorquin]

I can't add labels (no write access), a maintainer will need to add patch. The codecov checks are failing but the actual changes shouldn't reduce coverage.

[CodyCBakerPhD]

@h-mayorquin Perhaps worth an NWB Inspector check as well?

[h-mayorquin]

@h-mayorquin Perhaps worth an NWB Inspector check as well?

Yes:
NeurodataWithoutBorders/nwbinspector#697

[CodyCBakerPhD]

LGTM

Thanks for the catch

[h-mayorquin]

what's up with the code coverage?

[CodyCBakerPhD]

what's up with the code coverage?

Something to do with how it counts import lines

All source code lines look to be hit