os.Chmod cannot revoke read access on Windows — it only toggles the
read-only attribute — so the findImageInFolder spec opened the file
happily and failed there, and the upload spec passed for the wrong
reason: outcomeFailed came from the 1-byte payload failing to decode,
not from the source being unreadable.
findImageInFolder takes an fs.FS, so the failure is now injected and
the spec is filesystem-independent. The upload path goes through
os.Open directly and has nothing to inject, so it skips on Windows
rather than pretend to cover it.
ca8f4be369