Get all strings from resourcemanager

I need to write a program, that reads all string resources from dll and insert them into some table. I have the method, that reads resources:

    private static IEnumerable<KeyValuePair<string,string>> getAllResources(ResourceManager resourceManager, 
        Language language)
    {

        ResourceSet resourceSet = resourceManager.GetResourceSet(getCulture(language), true, true);

        IDictionaryEnumerator dictNumerator = resourceSet.GetEnumerator();

        // Get all string resources
        while (dictNumerator.MoveNext())
        {
            // Only string resources
            if (dictNumerator.Value is string)
            {
                var key = (string)dictNumerator.Key;
                var value = (string)dictNumerator.Value;
                yield return new KeyValuePair<string, string>(key, value);
            }
        }
    }

But when I started using it, I noticed that it also reads the resources, that added like a file (reads file content)

How can I ignore resources that are added as a "file", and read only strings?