Upload a file and ensure the parent directories exist. If the file exists, it will be overwritten.

Any request header of the form X-Metadata-<key>: <value> is persisted as a user-defined extended attribute on the uploaded file. The X-Metadata- prefix is stripped and the remaining header name is lowercased to form the metadata key; the resulting map is returned on EntryInfo lookups (e.g. Stat, ListDir). Each upload replaces the file's metadata with the keys provided in that request: keys previously stored but absent from the new request are removed, and an upload that sends no X-Metadata-* header clears all existing metadata. Both keys and values must be printable US-ASCII (bytes 0x20-0x7E) and are rejected with HTTP 400 otherwise. Each key is capped at 246 bytes (the Linux VFS xattr-name limit minus the namespace prefix), and the combined size of all metadata on a file (keys plus values, with the namespace prefix counted per key) is capped at 4096 bytes to stay within the filesystem's per-inode xattr budget. Multiple files in a single multipart upload receive the same metadata. If the same X-Metadata-<key> header is sent more than once, only the first value is used.