Support URLFile in the upload_file function

[aron]

We would like predict functions to be able to return a remote URL rather than a local file on disk and have it behave like a file object. And when it is passed to the file uploader it will stream the file from the remote to the destination provided.

class Predictor(BasePredictor):
    def predict(self, **kwargs) -> File:
        return URLFile("https://replicate.delivery/czjl/9MBNrffKcxoqY0iprW66NF8MZaNeH322a27yE0sjFGtKMXLnA/hello.webp")

This PR adds an additional check to the upload_file() handler to call fh.seekable() before attempting to seek. This allows instances of io.IOBase that do not support seek (like URLFile) to be uploaded.

We also add a name attribute to URLFile. This is used by the upload_file function to infer the file extension and mime type.

[nickstenning]

Other than the filename extraction which doesn't really work for URLs in general, this LGTM.

[nickstenning]

I think we need to limit this to http/https/data URLs for security reasons, as noted in #1987.