Add apng extension to png plugin

[ghost]

Fixes:

Some apps rely on file extensions. At least they can open apng as png now. Until apng support is actually added #3483

Changes proposed in this pull request:

• simply add the apng extension in the png plugin.

[ghost]

-_-...

[hugovk]

@pirate486743186 Please can you add a test .apng file and a test case?

[ghost]

i'm a total noob... it got more complicated then i thought it would be... I think i did what you want.

1)i added a copy of a converted iss634.webp to apng
2)i adapted a version of test_load_verify for apng
3) Merged in one commit.

[timgraham]

I wonder if this is working correctly. After upgrading to Pillow 5.4.0 a test failure appears in Django:
https://github.com/django/django/blob/14e2b1b065085c1d2d3e94ebaeefe25e12595a00/tests/forms_tests/field_tests/test_imagefield.py#L25-L38
On the last line, the content_type has changed to image/apng. I'm not sure what differentiates png and apng but the test file is a 1x1 pixel that's not intended to be animated as far as I know.

[radarhere]

Thanks for finding this. I've created PR #3525 to resolve the situation.