Upload Dialog - Show error message if extensions is not allowed

Currently, you can select a file with a non-supported extension and submit upload.
It is treated identically to an allowed extensions with the difference that it won't be uploaded by the backend.

UploadDownloadFile needs to incorporate a check and display an error message in the dialog if extension is not supported. User shouldn't be able to submit upload with invalid extension.

找到 UploadDownloadFile 元件，並檢查提交前如何處理所選檔案的副檔名。使用不受支援的副檔名重現上傳，然後加入對話框錯誤，並阻止使用無效副檔名的提交。確認受支援的副檔名仍可正常上傳，而不受支援的副檔名會在 UI 中被拒絕。