Detect mimetypes

[radarhere]

#3501 (comment) reports that one of Django's tests is currently failing, as a call to Pillow from Django - Image.MIME.get(image.format) - is reporting that PNG has mimetype 'image/apng'.

A simple way to replicate this in just Pillow is

>>> from PIL import Image
>>> Image.preinit()
>>> Image.MIME.get('PNG')
'image/apng'

This is because

Pillow/src/PIL/PngImagePlugin.py

Lines 910 to 911 in e32e3fd

	Image.register_mime(PngImageFile.format, "image/png")
	Image.register_mime(PngImageFile.format, "image/apng")

calls register_mime twice

Pillow/src/PIL/Image.py

Lines 2806 to 2814 in e32e3fd

	def register_mime(id, mimetype):
	"""
	Registers an image MIME type. This function should not be used
	in application code.
	:param id: An image format identifier.
	:param mimetype: The image MIME type for this format.
	"""
	MIME[id.upper()] = mimetype

So the dictionary keeps the information from the last call.

However, this problem does not just exist for PNG. It also exists for Jpeg2K and SGI. This PR solves this by changing to just one register_mimetype call and overriding that default by setting ImageFile.custom_mimetype when appropriate, to be returned by the ImageFile.get_format_mimetype() method.

This PR will fix the Django test as it currently exists.

• Regarding PNG, I have added code to detect whether an image is APNG or not.
• Regarding SGI, http://paulbourke.net/dataformats/sgirgb/sgiversion.html -

On SGI systems, SGI image files end with the extension .bw if they are B/W images, they end in .rgb if they contain RGB image data, and end in .rgba if they are RGB images with alpha channel.

Sometimes the .sgi extension is used as well.

Presuming that the mime type should follow suit, and given that indeterminate description, my suggestion is that 'image/rgb' be used for RGB image data, and 'image/sgi' be used otherwise.

• Regarding Jpeg2K,
  http://jhove.openpreservation.org/modules/jpeg2000/ -

The File Type box BR field is "jpx " (note the trailing SPACE character)

The additional test image is taken from https://raw.githubusercontent.com/openpreserve/format-corpus/master/jp2k-formats/balloon.jpf, which is CC0 according to https://github.com/openpreserve/format-corpus