OP solved it, closing

Don't work if n is non digit!
with float may return an error!

I guess original tests were not so harsh

if n is anything other than an integer or integer-formatted string (e.g. '123') that is >=0, throw a TypeError

Read the problem description.

This does not correctly handle floats.

mind elaborate?

trash low school mathematics or physics needed, no programming. Find a formula only that you can't know by your own.

Re-read the Kata description properly:

Assume it will start with date represented as long number
Followed by an underscore
You'll have then a filename with an extension

The delimiter between the data and the filename is a single underscore. If there are multiple underscores, then naturally the latter underscores are not part of the delimiter.

Read this: https://github.com/codewars/codewars.com/wiki/Troubleshooting-your-solution

This is not a way to report an issue, do it properly next time.

trash broken invavid kata

Testing for '84199869583086393_-RZQDc_G_DBUlxQP_Apzk_oLLcpclD.96.ZGBzqOf1K457B5355d36888h24P'
It should work for random inputs too: 'RZQDc_G_DBUlxQP_Apzk_oLLcpclD_.96_' should equal '-RZQDc_G_DBUlxQP_Apzk_oLLcpclD.96'
(damn markdown not shows as it is - '-__filename.extension')

HOW?!
"It should work for random inputs too: 'u.5t8' should equal '-_u.5t8' "

Once i random passed, but not submitted...
And no more luck.
Tried hard to correct to this: "(-|__)?([a-zA-Z][a-zA-Z_-]*.[a-zA-Z-\d]+)?\w?"

until:
Server Execution Error:
Too Many Requests
Request Error:
Request failed with status code 429
Please fix the issue and try again.