Issue regarding file extension filtering in uploading

**Describe the bug**
In code [here](https://github.com/HumanSignal/label-studio/blob/a0b30e92e0d677143f85bc3c6eb8030cf1bf14d9/label_studio/data_import/uploader.py#L61), it uses the user provided file name. It might be bypassed by manipulating the file extension. Using methods like checking magic code instead of relying on extension would be better.