feat(up,uploader): add --video/--audio flags and forced media type handling

[Andre-cc]

添加强制设置文件类型的参数，解决视频文件上传会被识别为音频文件的问题

[XMLHexagram]

First of all, if you want to add the force video/audio feature, please follow the code style of the existing AsPhoto implementation. Add AsVideo() and AsAudio() to the type Elem interface in core/uploader/iter.go, and use them properly in uploader.go.

Second, I'm uncertain about the thing you described where a video might be misidentified as an audio. In my opinion, judging by reading the MIME type should not lead to such misidentification. Therefore, could you please provide a file that can reproduce this issue?

Finally, could you please respond in English? While the project doesn't force you using English in PRs, this requirement exists for issues. Therefore, I believe it would be best to maintain language consistency. If you're not confident in your English level, feel free to use any translation tools.