Gets or sets a value that indicates
whether ASP.NET routing should handle
URLs that match an existing file.

这是我从here读到的内容：

However,the routing system still does check the file system to see if an
incoming URL happens to match a file or disk,and if so,routing ignores the request (bypassing
any route entries that the URL might also match) so that the file will be served directly.
This is very convenient for static files,such as images,CSS,and JavaScript files. You can
keep them in your project (e.g.,in your /Content or /Script folders),and then reference and
serve them directly,just as if you were not using routing at all. Since the file genuinely exists
on disk,that takes priority over your routing configuration.

If,instead,you want your routing configuration to take priority over files on disk,you can set the RouteCollection’s RouteExistingFiles property to true. (It’s false by default.)