Uploading `.part` files (from another program)

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Is your feature request related to a problem? Please describe.
We use a program that creates files with a .part extension, we would like to upload those files to Nextcloud.
Unfortunately the .part extension is forbidden and is hardcoded here.

(This program is not supported anymore and we can't change the extension of the files generated)

Describe the solution you'd like

1. Having a system that change the extension of incoming files to something random and translate it to the original extension for the outgoing files
2. A way to allow .part extension -> By overiding it in the config for example
3. Using a really random extension (a uid for example)
4. Allow users to configure a specific extension in place of ".part"

Describe alternatives you've considered
I tried to make an app that does the solution n° 1. That's how I discovered that the .part was hardcoded inside the getForbiddenExtensions()

Additional context
I'm willing to do a pull request and help make this feature. But I have no idea of how big of a change this is. However I think the solution n°1 I described would be a good solution, as it would completly negate the problems with sending files like .htaccess or *.part and so on.

Start with lib/private/Files/FilenameValidator.php and its getForbiddenExtensions() method, which currently hardcodes the forbidden .part extension. Compare the proposed configuration and renaming approaches, then clarify the desired behavior and security constraints before defining what a complete change would need to preserve.