Internet Explorer handles downloads from FILE:// links differently from downloads using HTTP:// links. You can use this difference, in conjunction with the Multipurpose Internet Mail Extension (MIME) type settings on the server, to automatically receive a Save As dialog box instead of having the file downloaded directly into the browser.

Internet Explorer 3.x

When you use Internet Explorer 3.x to download a document from a FILE:// link, Internet Explorer downloads the file and reads its extension for the MIME type. If the extension is registered, Internet Explorer checks the registry for that file type and performs the action required by that file type. If the extension is not registered, Internet Explorer prompts you to determine if you want to download the file.

Internet Explorer 4

Internet Explorer 4 behaves in the same way as Internet Explorer 3.x for file types with registered extensions. If the extension is not registered and you are downloading the file from a FILE:// link, Internet Explorer tries to determine the type of file that it is downloading on its own. However, if the file you are downloading contains plain text (but has an extension not associated with text files), Internet Explorer 4 detects that it is a plain text file and downloads it into the browser so you can view it.

When you use an HTTP:// link to download a file, the file type is associated correctly because the MIME type (file type) is sent by the server. Therefore, the browser does not try to determine the file type on its own from the file contents.