open_file.js

function openFile() {
    var file = {
        fileContent: "bXkgc2VjcmV0IGNvbnRlbnQ=", //Contents of the file. 
        fileName: "example.txt", //Name of the file.
        fileSize: 24, //Size of the file in KB.
        mimeType: "text/plain" //MIME type of the file.
    }

    Xrm.Navigation.openFile(file, 2);
}