The Selenium attach_file method allows browsers running on remote machines to upload files to a test application. This is achieved by using the `attach_file` method, which takes two parameters: a locator for the file input field and a URL pointing to the desired file. The Selenium RC server downloads the file to a temporary location on the local machine and then attaches it to the specified field. This method is supported only in Firefox browsers. To use this feature, files must be placed in a public URL or accessed using HTTP Auth. The method has some limitations, including the requirement that the file be at the top level of the URL, but Sauce OnDemand has implemented workarounds for these issues, making it easier to upload files in certain configurations.