true to overwrite the existing item if it exists when uploading to SharePoint server, otherwise; false. Default value is true.
If the value of OverwriteExistingFiles is false and item with same name as the uploaded item already exists in the SharePoint server folder, an exception will be thrown.
If the value of OverwriteExistingFiles is true and item with same name as the uploaded item already exists in the SharePoint server folder, then the existing item will be overwritten.
For an example, refer to SharePointClient.