Handle URL `Path` in `cog predict`
On replicate.com, a model with return type `Path` can return a URL, and it is handled as though the model returned a local file path.
Locally, `cog predict` will fail with
```
ⅹ Failed to write output: Failed to decode dataurl: missing data prefix
```
This change makes `cog predict` download and save the output URL.
The `useExtensionIfUnknownContentType` is mostly needed because .webp is incorrectly returned by replicate.delivery as application/octet-stream.
