mirror does not preserve Content-Type when copying objects between S3 backends

[makhomed]

When mirroring objects between two S3-compatible backends with:

rc mirror stage/bucket prod/bucket

the destination object loses its original Content-Type.
Example: a source object with Content-Type image/jpeg becomes
application/octet-stream on the target.

From the code, mirror currently does:

• source_client.get_object(&source_full)
• target_client.put_object(&target_full, data, None)

So the upload is performed without passing Content-Type at all.

S3Client::head_object() already reads content_type() into ObjectInfo,
so a minimal fix would be to fetch source metadata first and pass
source_info.content_type.as_deref() into put_object().

This likely affects not only Content-Type but potentially other object metadata as well.