Restrict upload by filetype or mimetype using Dexterity on Plone - plone

Question

I have a custom content type, built with dexterity. In the schema (The schema is listed below), I use 'plone.namedfile.field.NamedFile' for attachements/uploads.
I would like to restrict uploads so that only mp3 files can be attached to my content type. What is the best approach for achieving this?
Here is the full schema/model for my content type:
<model xmlns="http://namespaces.plone.org/supermodel/schema">
<schema>
<field name="date" type="zope.schema.Date">
<description />
<title>Date</title>
</field>
<field name="speaker" type="zope.schema.TextLine">
<description />
<title>Speaker</title>
</field>
<field name="service" type="zope.schema.Choice">
<description />
<title>Service</title>
<values>
<element>1st Service</element>
<element>2nd Service</element>
</values>
</field>
<field name="audio_file" type="plone.namedfile.field.NamedFile">
<description />
<title>Audio File</title>
</field>
</schema>
</model>
I shall begin my search here: http://plone.org/products/dexterity/documentation/manual/developer-manual/reference/default-value-validator-adaptors

Answer 1

I've decided to use javascript for my first line of validation.
I've based my solution on information found at <input type="file"> limit selectable files by extensions
Based on the advice my script looks something like this:
$(document).ready( function() {
function checkFile(event) {
var fileElement = document.getElementById("form-widgets-audio_file-input");
var fileExtension = "";
if (fileElement.value.lastIndexOf(".") > 0) {
fileExtension = fileElement.value.substring(fileElement.value.lastIndexOf(".") + 1, fileElement.value.length);
}
if (fileExtension == "mp3") {
return true;
}
else {
alert("You must select a mp3 file for upload");
return false;
}
}
$("form#form").bind("submit",checkFile);
});
This is half the solution, next I'll need to add validation on the server side.