Right, but this is the blueprint for your posts. And it looks as if you want to select a cover image for your post. Therefore, you can either use a files section like you are currently doing, or maybe even better a files field. In any case, this section/field would then show the selected image.
Therefore, I do not understand the purpose of your query in this context. The query for a files section or field would have to return a files collection, i.e. multiple files, not a single file.