fix(remote_storage/s3): forward entry.Attributes.Mime as ContentType
filer.remote.sync was uploading every object without a Content-Type, so
S3-compatible backends (e.g. Backblaze B2) stored binary/octet-stream
and browsers refused to render HTML, CSS, etc.
Pass entry.Attributes.Mime through to UploadInput.ContentType, leaving
the header unset when no Mime is recorded so the remote keeps its own
default behavior.
629beda1eb