Nextcloud app uploads sent application/octet-stream as Content-Type,
causing images to not be recognized. Now both WebDAV PUT and chunked
upload paths call refine_content_type() which detects via magic bytes,
then extension, then falls back to the client header. Also fixes
update_file() which previously hardcoded application/octet-stream.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>