You can use $last variable within ng-repeat directive. Take a look at doc.
You can do it like this:
<div ng-repeat="file in files" ng-class="computeCssClass($last)">
{{file.name}}
</div>
Where computeCssClass is function of controller which takes sole argument and returns 'last' or null.
Or
<div ng-repeat="file in files" ng-class="{'last':$last}">
{{file.name}}
</div>