Do not alter the filename extension guessed from a Note

ext = ".txt"
mime_type = props.get("note:mime_type")
if mime_type == "text/html":
    ext = ".html"
elif mime_type == "text/xml":
    ext = ".xml"
elif mime_type == "text/x-web-markdown":
    ext = ".md"
name = props["dc:title"]
if not name.endswith(ext):
    name += ext