For those of you encountering issues reading files on Android 10, you should try loading your app into this test server and test it with the corresponding companion.
In this version, the File component gains a LegacyMode property. You will need to set this property to true in order to read files from outside the app-specific directory.
Note, this will only work for another year or so. Starting with Android 11, Google will no longer allow apps to read outside of their app-specific directories. You should read "Migrate data to directories that are visible when using scoped storage" to understand the implications of what you need to do. App Inventor cannot migrate your data for you, but this flag will allow you time to update your app over the coming year to be compatible. If you fail to migrate, an upgrade to Android 11 may cause your app to stop working and there will be nothing we can do for you.