function createUntitledDocument(counter, fileExt) {
        var filename = Strings.UNTITLED + "-" + counter + fileExt,
            fullPath = _untitledDocumentPath + "/" + filename,
            now = new Date(),
            file = new InMemoryFile(fullPath, FileSystem);

        FileSystem.addEntryForPathIfRequired(file, fullPath);

        return new DocumentModule.Document(file, now, "");
    }
Creates an untitled document. The associated File has a fullPath that
looks like /some-random-string/Untitled-counter.fileExt.

@param {number} counter - used in the name of the new Document's File
@param {string} fileExt - file extension of the new Document's File, including "."
@return {Document} - a new untitled Document