As calvo_ already mentioned, Alfresco only supports document-level metadata, tags etc. This is because Alfresco actually does not care about the type of the file that you put in Alfresco. Yes, it has some specific support for common types, but in general you can put anything inside it, even ISO images. How should/would Alfresco support tagging anything in the content of a "document" if that "document" can literally be anything from text file to binary?
