Assuming that the page that contains the example code is two directories deep from the root of the site e.g.
http://www.example.com/one/two/example-page.php
then you could do this:
<a href='http://<?php echo htmlentities($_SERVER['SERVER_NAME'], ENT_QUOTES)?>/files/uploaded_file.jpg'>Your Uploaded File</a>
If that assumption is wrong and the files are stored in a directory that is at least one directory deeper than the root e.g.
http: //www.example.com/content/files/uploaded_file.jpg
then you have to a little more work to get the absolute url.
<?php
$FilesystemPath = str_replace("\\", "/", realpath(dirname(__FILE__) . "/../../files/")) . "/uploaded_file.jpg";
$DocRoot = $_SERVER['DOCUMENT_ROOT'];
$Uri = str_replace($DocRoot, '', $FilesystemPath);
?>
<a href='http://<?php echo htmlentities($_SERVER['SERVER_NAME'] . $Uri, ENT_QUOTES)?>'>Your Uploaded File</a>
This example assumes that you know that the files directory is in the directory that is two directories up from the page's directory.
All of these example assume a one to one mapping of directories on the file system and there's no url rewriting going on. Also I'm assuming you are using just plain http and not https. If you want to present links in the same protocol as what the page is served under then that's a separate question.