code: private boolean extensionValidForContentType(String fileName) {
  		boolean valid = false;
  
  		IContentType type = getContentType();
  		// there is currently an extension
  		if (fileName.lastIndexOf('.') != -1) {
  			// check what content types are associated with current extension
  			IContentType[] types = Platform.getContentTypeManager().findContentTypesFor(fileName);
  			int i = 0;
  			while (i < types.length && !valid) {
  				valid = types[i].isKindOf(type);
  				++i;
  			}
  		} else
  			valid = true; // no extension so valid
  		return valid;
  	}
docs: Verifies if fileName is valid name for content type. Takes base content
  type into consideration.
  
  @param fileName
  @return true if extension is valid for this content type