[BUG]extensions.py extension

[yulk]

When the uploaded file is all in Chinese, such as "图片.png", the extension function in this file flask_uploads\extensions.py. will return empty.

Maybe you should modify like this:

def extension(filename: str) -> str:
    ext = os.path.splitext(filename)[1] if len(os.path.splitext(filename)[1]) >0 else os.path.splitext(filename)[0]
    if ext.startswith('.'):
        # os.path.splitext retains . separator
        ext = ext[1:]
    return ext

[jugmac00]

Thank you for the report. Would you be able to create a pull request with the fix? And ideally a test case which shows the bug.