Expose registered file extensions

[lggomez]

Is there any way of getting the file extensions supported by the library?
Usually it's a good practice to expose the supported types or extensions, specially in libraries that handle files, so the consumer can have a way to obtain or check its target extensions based on the ones provided by the library

I'm not very familiar with Pillow's code but this method in Image.py should do the trick:

def registered_extensions():
    """
    Returns a dictionary containing all file extensions belonging
    to registered plugins
    """
    if not bool(EXTENSION):
        init()
    return EXTENSION

Anyway, it's just a suggestion so you shall take it with a grain of salt. Keep up the good work on this fork

[radarhere]

From my understanding, preinit() should not be necessary when immediately followed by init(). It's just a version of init() that aims to be faster by only covering the most popular formats.

[lggomez]

Indeed, after checking I see that using init() will be enough, I corrected it

Do you consider this a worthy addition? If so I can make the pull request

[radarhere]

Potentially, yes. I won't be the one to ultimately merge it, but you should certainly feel free to create a PR as a way of moving this forward.

[radarhere]

This has been done in #2343.