Iterate over file paths within root filtered by specified extensions.
    :param compat.string_types root: Root folder to start collecting files
    :param iterable exts: Restrict results to given file extensions
    :param bool recursive: Wether to walk the complete directory tree
    :rtype collections.Iterable[str]: absolute file paths with given extensions

def iter_files(root, exts=None, recursive=False):
    """
    Iterate over file paths within root filtered by specified extensions.
    :param compat.string_types root: Root folder to start collecting files
    :param iterable exts: Restrict results to given file extensions
    :param bool recursive: Wether to walk the complete directory tree
    :rtype collections.Iterable[str]: absolute file paths with given extensions
    """

    if exts is not None:
        exts = set((x.lower() for x in exts))

    def matches(e):
        return (exts is None) or (e in exts)

    if recursive is False:
        for entry in compat.scandir(root):
            if compat.has_scandir:
                ext = splitext(entry.name)[-1].lstrip('.').lower()
                if entry.is_file() and matches(ext):
                    yield entry.path
            else:
                ext = splitext(entry)[-1].lstrip('.').lower()
                if not compat.isdir(entry) and matches(ext):
                    yield join(root, entry)
    else:
        for root, folders, files in compat.walk(root):
            for f in files:
                ext = splitext(f)[-1].lstrip('.').lower()
                if matches(ext):
                    yield join(root, f)