When attempting to add an attachment to a service, the request is denied and returns a "403 - Forbidden" error.
The viewer is attempting to proxy a host that is not recognized, causing the proxy.ashx to throw the 403 error.
Resolution: The ArcGIS server URL in question must be added to the proxy.config file to ensure the proxy is forwarding appropriately.
Edit the HTML5 Viewer proxy.config file and add the ArcGIS Server address like the example below.
Within the <serverUrls> blocks add the below example with your ArcGIS Server address. Please note, the ArcGIS Server must support Anonymous Access.
<serverUrl url="http://services.arcgisonline.com" matchAll="true"/>