FileSystem has been added to permissions, chrome.fileSystem is still undefined

problem description

when I am working on the chrome extension, I need to use the api, of fileSystem to add the fileSystem permission to permissions, but at run time, chrome.fileSystem is still undefined, which is the configuration of permissions

    "permissions": [
        "contextMenus",
        "tabs",
        "webRequest",
        "webRequestBlocking",
        "storage",
        "system.display",
        "fileSystem"
    ],

Why is this?

later I used the native fileSystem api, of html5 because I wanted to persist the data and could not be cleared by the operation of clearing the browser cache, so I wrote the following code in the background.js of the plug-in,

    window.requestFileSystem = window.requestFileSystem || window.webkitRequestFileSystem;
    navigator.webkitPersistentStorage.requestQuota(bytes, function (grantedBytes) {
        alert(":" + grantedBytes);
        window.requestFileSystem(window.PERSISTENT, grantedBytes, initFs, errorHandler);
    }, errorHandler);

as a result, the grantedBytes value in the callback function is 0.
does it mean that plug-ins can"t apply for presistent space?

Mar.21,2022
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