Image pipeline should allow to decide whether to auto-convert to jpg or not

[ddebernardy]

Not 100% it's accurate, but the issue got reported here:

https://news.ycombinator.com/item?id=10941213

Image pipeline converts to jpg automatically. Cannot be disabled without writing all the image code yourself

[nramirezuy]

I guess we refer to this line

[nramirezuy]

@dangra I set this to discuss because I'm not sure if there is any valid reason to force 'JPEG'.

[redapple]

something along thoses lines was proposed in #250
but there was no followup since

[djunzu]

One way to disable auto-convert to .jpg is to use FilesPipeline. The only drawback is that another features from ImagesPipeline can't be used with this approach.

It makes sense that someone may want to use the features provided by ImagesPipeline but not auto-convert. I think a good solution that maintains backward compatibility is to provide a IMAGES_CONVERT_TO option that defaults to JPG and accepts NONE. And it could also allows to convert to another formats...