[MEDIA] ImageFile now extends MediaFile and validates images more aggressively.

[MEDIA] ImageFile now extends MediaFile and validates images more aggressively.
Default supported files need to use consistent names. Bumped version to 1.20.0

ImageFile has been changed to extend MediaFile and rely on it to partially
validate files. This validation has been extended to not rely solely on
Fileinfo, as it is disabled on some places. Now it'll try to use the shell
command `file`, if Fileinfo isn't available.

ImageFile now converts every new upload to PNG, except JPEG and GIF, which
are kept, but still resized (to the same size), to remove possible scripts
embedded therein.

MediaFile::fromUpload will return an ImageFile if the uploaded file is an image
or a MediaFile otherwise.

MediaFile can be constructed with an id with value -1 to denote a temporary
object, which is not added to the DB. This is useful to create a temporary
object for representing images, so it can be used to rescale them.

The supported attachment array needs to be populated with the result of calling
`image_type_to_extension` for the appropriate image type, in the case of images.
This is important so all parts of the code see the same extension for each image
type (jpg vs jpeg).

Added documentation to classes/File.php and to lib/MediaFile and lib/ImageFile