Does search query string match start of file extension?

Thanks for the quick response.

Would in fact be good to have a documentation how matching of the query string happens. This is not very clear:

The string to search for. May match across multiple fields based on the request arguments. Query string may be rewritten to improve relevance of results.

My tests suggest that the start of each token matches where a token means a new word, a title case word or a number. E.g. for "My WordFile123.docx" things like "my", "wo", "fil", "12", and "d" match. But like you say it's not ideal to rely on something that's not guaranteed to work in the future.