The point of the file input is to provide a file. Names that come with it are "whatever the browser vendor feels like using", they aren't guaranteed to have anything to do with the file name on the file system at all.
You can't change what the browser sends.
If you are going to make use of the name sent by the browser, then you need to make sure it is valid for whatever you are going to do with it (e.g. make sure it only includes characters that are allowed in filenames on your filesystem). This makes it something that must be handled on the server (just like any other client supplied data).
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