[when Image type will exist] provide a way to get the data as binary + filename
@severo, maybe somehow related to this PR ?
- #3129 @severo I'll keep that in mind.

You can track progress on the Image feature in #3163 (still in the early stage).  
**Is your feature request related to a problem? Please describe.**

When a dataset cell contains a value of type Image (be it from a remote URL, an Array2D/3D, or any other way to represent images), I want to be able to write the image to the disk, with the correct filename, and optionally to know its mimetype, in order to serve it on the web.

Note: this issue would apply exactly the same for the `Audio` type.

**Describe the solution you'd like**

If a "cell" has the type `Image`, provide a way to get the binary content of the file, and the filename, eg as:

```python
  filename: str
  data: bytes
```

**Describe alternatives you've considered**

A way to write the cell to the disk (passing a local directory), and then return the pathname, filename, and mimetype.
