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.

定位 UploadDownloadFile 组件，并检查提交前如何处理所选文件的扩展名。使用不受支持的扩展名复现上传，然后添加对话框错误，并阻止使用无效扩展名的提交。确认受支持的扩展名仍能正常上传，而不受支持的扩展名会在 UI 中被拒绝。