Why does secure_filename() remove a leading underscore?

[Siecje]

Why does werkzeug.utils.secure_filename() remove a leading underscore? Is it safe to add it back after?

>>> from werkzeug import utils
>>> utils.secure_filename('_why.py')
'why.py'

werkzeug/tests/test_utils.py

Line 298 in d129d17

assert utils.secure_filename('__filename__') == 'filename'

[davidism]

Seems to be for normalization purposes. You can do whatever you want to the filenames, but evaluating their security is up to you.