Assert image type

[radarhere]

This is part of #8362 - I'm hoping to break down that PR into easier-to-review chunks.

The test suite contains code like

Pillow/Tests/test_file_apng.py

Lines 14 to 15 in 0e3f51d

	with Image.open("Tests/images/apng/single_frame.png") as im:
	assert not im.is_animated

But is_animated is not a property on a generic, only certain formats. So this PR adds asserts to make sure the image are of the right format.