Earlier quoted context omitted.
To clarify our views: The words and sentences are inside the file. The filenames are identifiers to the files. Identifiers' purpose is mainly to identify, rather than to communicate. On the other hand, we could imagine a user interface that users don't see (actual) filenames at all.
Identification is a form of communication. The identifier assigned to me at birth contains two spaces. Most people use one of the shorthand forms, but still.
That is, I think it is safe to say that we expect we can include spaces in the names of things. The movie is named "Star Wars", after all. That seems natural.
For people, though, we have grown used to someone having multiple parts of their name separated by a space. If given out of order, a comma.
Even in the "naming things" category. We are used to having to put quotes around names that have spaces. Or otherwise using a convention like Capitalized Identifiers to show something goes together. Probably isn't a clearcut rule.