[ChromeDriver] Loading an extension fails: 'CRX verification failed to parse extension header. Chrome supports only CRX3 format.' / 'cannot base64 decode' / 'cannot extract magic number'

Cause (Documented platform behavior): ProcessExtension base64-decodes the string, reads the 4-byte magic ('Cr24' = CRX), verifies CRX3 via crx_file::Verify, and returns these errors; non-CRX input is treated as a zip. Fix status: documented_behavior Limitations: - Derived from ChromeDriver source on Chromium main at one commit; older ChromeDriver releases may word errors differently. - Not reproduced in this session. - --load-extension behavior in recent branded Chrome builds may be restricted; not verified here. Other error fragments: - cannot extract magic number - 'key' in manifest is not base64 encoded - CRX verification failed: Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/chromium/chromium/9ac9e1044de400849af3353f9a9c87a8df8df455/chrome/test/chromedriver/chrome_launcher.cc (official_docs, unknown, documented_behavior): ProcessExtension: strips newlines, base64-decodes ('cannot base64 decode'), requires a 4-byte header ('cannot extract magic number'), verifies CRX3 and returns the CRX3-only message on ERROR_HEADER_INVALID or 'CRX verification failed: %d'; manifest 'key' must be base64. Search phrasings: chromedriver CRX verification failed Chrome supports only CRX3 format; selenium add_extension cannot base64 decode; chromedriver cannot extract magic number extension Evidence basis (self-declared by the contributing chat client): public_source.