val repo = setupRepo()
        val branch = "main"
        val file = tempFile.apply { writeText("test_data") }
        val path = "test_obj"
        val metadata = mapOf("foo" to "bar")
        val headers = mapOf("Accept" to "application/json",
                            "Content-Type" to "application/octet-stream") + metadata.mapKeys { (k, _) -> "$LAKEFS_METADATA_PREFIX$k" }
        val resourcePath = "/repositories/${repo.id}/branches/$branch/objects"
        val queryParams = URI(hashMapOf("path" to path).encode())
        testClient.updateParamsForAuth(arrayOf("basic_auth"), null, headers, null, resourcePath, "POST", queryParams)
        val stats = testClient.objectsApi.uploadObject(repo.properties!!.id, branch, path).content(file).execute()
        assert(stats.path == path)
        assert(stats.mtime <= System.currentTimeMillis())
        assert(stats.sizeBytes!! == file.length())
        println(stats.metadata)
        //        assert(stats.metadata == metadata)
        assert(stats.contentType == "application/octet-stream")

stats.metadata

headers

{Accept=application/json, Content-Type=application/octet-stream, x-lakefs-meta-foo=bar}

queryParams

?path=test_obj

stats

class ObjectStats {
    path: test_obj
    pathType: object
    physicalAddress: local:///home/elect/lakefs/data/block/tmp/junit15669663127659863511/data/ghkntbipdakrls40rqlg/cnios1ipdakrls40rqr0
    physicalAddressExpiry: null
    checksum: 6af8307c2460f2d208ad254f04be4b0d
    sizeBytes: 9
    mtime: 1709542918
    metadata: {}
    contentType: application/octet-stream
    additionalProperties: null
}

The Python wrapper has explicit code which makes this happen.

val storageNamespace by lazy { "local:/$tempFolder" }