Provide more metadata in Rows responses: URL + media type
If a cell contains text, we want to:
- [ ] know if it's an URL
- [ ] know if it's a relative path to a local file (ie: file_name column in metadata.csv file)
- [ ] if so: what is the MIME-TYPE of the resource (we can detect from the extension, or try some HEAD requests, or maybe *datasets* has a way to get that info, from the Feature)

It would be useful to transform URLs to `<a>` elements in the dataset viewer, and to show images and audio players if the URLs are media.
Provide more metadata in Rows responses: URL + media type: If a cell contains text, we want to:
- [ ] know if it's an URL
- [ ] know if it's a relative path to a local file (ie: file_name column in metadata.csv file)
- [ ] if so: what is the MIME-TYPE of the resource (we can detect from the extension, or try some HEAD requests, or maybe *datasets* has a way to get that info, from the Feature)

It would be useful to transform URLs to `<a>` elements in the dataset viewer, and to show images and audio players if the URLs are media.