I think when add/change the interceptor stack; the default interceptor stack will get removed.
Try adding after the <interceptor-ref name="uploadContent"> tag.
1. Your validation file name must be <actionclass>-<validation>.xml
2. your action should have getters and setters for the field against you are validating.
3. In interceptor basicStack must be above the validation & workflow interceptor in struts.xml.
4. <S:fielderror> is doesnot have a property "fieldName"
use like this
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