GitHub serves pasted attachments from /user-attachments/assets/<uuid>
with no file extension, so getImageExtension() silently defaulted to
".png". When the bytes are actually JPEG/GIF/WebP the downloaded file is
mislabeled, and the Read tool then sends a base64 image whose declared
media_type doesn't match its magic bytes — which the Anthropic API
rejects with `400 invalid_request_error` ("image was specified using the
image/png media type, but the image appears to be a image/jpeg image").
Sniff the real format from the buffer's magic bytes after download and
only fall back to the URL-based extension when the signature is
unrecognized. Adds a regression test for a JPEG at an extensionless URL.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
