The /stream/upload endpoint's overwrite flag is the x-prefixed header
x-override-name-exist, matching its siblings x-filename, x-parent_id and
x-streammode. uploadLargeFile sent the un-prefixed override-name-exist, which
the endpoint ignores, so overwriting a file of 10 MiB or larger created a
renamed duplicate instead of updating the existing file. The small upload API
passes the flag as a query parameter and is unaffected.