You may experience difficulties trying to upload files with ajaxified controls. For example using the standard on AJAX request the uploaded files items will be empty.
Indeed, there are browser restrictions and XmlHttpRequest object limitations which makes uploading files through AJAX requests impossible. Thus, if you want to use Telerik RadAjax controls with a page, where a control perform uploading files, this control should make regular post-backs instead.
In a typical scenario you may have the whole page (or a part of it) nested within an RadAjaxPanel. In this case you have to manually exclude from ajaxifying the control, which is implemented to upload files. Detailed information how this could be achieved may be found in the "Exclude controls from ajaxifying" help topic.
Using RadAjaxManager in a page, where files are uploaded, should not be a problem. You should only take care of not setting the correspondent control as ajaxified in Manager's AjaxSettings.
When you instantiate file upload control through ajax call (e.g. set Visible="true" or control is nested in template control) you will need to manually add enctype="multipart/form-data" to the page form tag as our ajax controls (ASP.NET as well) do not update the form tag.