* s3: answer application/x-directory for a directory without a stored mime
A real directory reached via a trailing-slash GET/HEAD answered the
octet-stream default when it had no stored mime, so Hadoop-style S3
filesystems (flink-s3-fs-presto and friends) classified the path as a
0-byte file instead of a directory and then failed reading it as one.
Answer application/x-directory, the marker type those clients probe
for. Stored mimes still echo verbatim, and a file promoted to a
directory keeps octet-stream for its data.
* s3: 404 GET and HEAD on a bare directory path consistently
A directory with no object data of its own answered differently per
path: plain GET gave an empty 200, ranged GET and non-versioned HEAD
gave 404, and on versioned buckets the null-version fallback adopted
the filer directory as a 0-byte object and answered 200. Clients that
probe HEAD-then-GET took the 200s at face value, treated the path as
an empty file, and never fell back to LIST-based directory discovery.
Answer 404 for a bare directory path everywhere, which is what AWS
returns for a prefix. A file promoted to a directory keeps its data
and stays retrievable.
