Fix hv.save for png images

[Guillemdb]

Closes #4302

Calling holoviews.save on panels ignores the parameter fmt and outputs all the layout objects as html files.

This PR updates the name of the file passed to plot.layout.save to include the file extension when the selected format is png. This allows panel to also export the panels as png images.

[Guillemdb]

AFAIK panel only supports html and png formats, so I added a check for the format to be value. If you want invalid formats to be handled differently please tell me and I will update it.

I am not familiar with the tests of the package. But If you point me to the place where you are testing the function I modified I can add a test to assert the ValueError is raised.

I can also change the message of the error if you have a better suggestion.

[philippjfr]

To be clear the intention was that the format would be specified as part of the filename and fmt would mostly be used to switch between different widget types, i.e. widgets or scrubber.

[philippjfr]

I think hv.save should probably check the filename extension and then validate that against the supplied format but that can be a separate PR.

[Guillemdb]

To be clear the intention was that the format would be specified as part of the filename and fmt would mostly be used to switch between different widget types, i.e. widgets or scrubber.

I have been using this wrong all the time 😅, I will modify the code of my libraries to reflect this. Thanks for the tip!

[philippjfr]

I have been using this wrong all the time 😅, I will modify the code of my libraries to reflect this. Thanks for the tip!

That's likely more of a failure of our docs, if you have suggestions on how and where to clarify this I'd be very open to make those changes.

[Guillemdb]

I think hv.save should probably check the filename extension and then validate that against the supplied format but that can be a separate PR.

Should I remove the ValueError?

If you send me a link with the valid formats (just to be sure I am not missing anything) I can make another PR with that.

[philippjfr]

Should I remove the ValueError?

I think the ValueError is fine tbh, that's all the formats that bokeh can export directly. Anything else is already handled in a different way.

[Guillemdb]

That's likely more of a failure of our docs, if you have suggestions on how and where to clarify this I'd be very open to make those changes.

I will think about it and make an informed suggestion (I hope in the form of a PR)