<p>I have a simple HTML upload form, and I want to specify a default extension ("*.drp" for example). I've read that the way to do this is through the ACCEPT attribute of the input tag, but I don't know how exactly.</p>

<pre><code>&lt;form enctype="multipart/form-data" action="uploader.php" method="POST"&gt;
Upload DRP File:
&lt;input name="Upload Saved Replay" type="file" accept="*.drp"/&gt;&lt;br /&gt;
&lt;input type="submit" value="Upload File" /&gt;
&lt;/form&gt;
</code></pre>

<p><strong>Edit</strong>
I know validation is possible using javascript, but I would like the user to only see ".drp" files in his popup dialog. Also, I don't care much about server-side validation in this application.</p>

The accept attribute expects MIME types, not file masks. For example, to accept PNG images, you'd need accept="image/png". You may need to find out what MIME type the browser considers your file type to be, and use that accordingly. However, since a 'drp' file does not appear standard, you
might
have to accept a generic MIME type.
Additionally, it appears that most browsers may not honor this attribute.
The better way to filter file uploads is going to be on the server-side. This is inconvenient since the occasional user might waste time uploading a file only to learn they chose the wrong one, but at least you'll have some form of data integrity.
Alternatively you may choose to do a quick check with JavaScript before the form is submitted. Just check the extension of the file field's value to see if it is ".drp". This is probably going to be much more supported than the accept attribute.