Fix written image-format

[olaf-mandel]

The default behaviour is to name all files as <basename>.jpg on the assumption that all images are downloaded with type image/jpeg. But this is not necessarily true, images may also be downloaded as image/png in which case the file-name is wrong.

To fix this, add a new choice native to the option --images-format and have it detect the actual downloaded image type from the Content-Type header. Make this new choice the (changed) default choice. Keep the choices jpg and png but enforce conversion in both cases if the downloaded image does not match the desired format.

[Zehina]

Thank you again @olaf-mandel for all the improvements you've submitted. I was looking at merging either before or after #36 since that changes the whole structure of the project.

This looks good to me so just to tackle how this is going to interact with #36 as that branch for now forces a single image-format to be used but does save the files corectly. My guess is that adding native as an option will just not add any AioImageFormatTransformer to the list of transformers.

• If you don't mind you can port your changes to a branch that targets new- structure which I think can relatively be easily done there as well. But using a different way.
• Or we can merge this for now and I'll work on porting it to new-structure.