What do you mean by "Open file" command? Do you enter the http:// URL into the address bar or do you do anything else? Only if your page is served via http:// the situation will be comparable to visiting your page through the website. If you use the "Open File" command, as it happens to display a file picker window, and is often accessed through CTRL+O, then the HTML page will not be served through HTTP and thus be able to access local resources through the filesystem.

Please make sure that you understand the difference between "filesystem" and "HTTP", and make sure your testcases are comparable.