use appframework for preview system

[georgehrke]

Migrate preview.png, etc ... to appframework.

Subtasks:

• Add a proper previewController
• Preview / Image Response types (require stream based responses)

[oparoz]

How will this work when we have to call several methods one after the other? We can't make REST calls for that as it would hurt performance badly.

[georgehrke]

@oparoz I'm not sure what you are referring to. What several methods do we have to call one after another?

[oparoz]

A Typical sequence

isMimeSupported()
getPreview()
valid()
width()
height()
isCached()

It's currently useful to be able to get an object so that we can call these methods.
I guess all the returned values could be included in the reply we get from the controller, if that reply was to include the values of all the public methods.

Sometimes we also use

resource()

But I guess it could be a separate call and not be too problematic as long as the Image class stays in core.

[oparoz]

• Move all non-core providers to apps
• Move mimetypes.list.php to a text file in /config Move mimetypes.list.php to a text file in /config #15384