How does one manage external resources in a SOLID way?

I'm currently working on a project where I'm parsing an external file to an abstracted object like so:

public interface IMapConverter
{
    IMap Convert(IMapFile file);
}

public interface IMapFile
{
    void Load(string filePath);
    string Json { get; }
}

My plan was to abstract the loading of the file by using the IMapFile implementation and creating the StreamReader inside that implementation. I would then pass that on to the IMapConverter.Convert method. Would that be a correct approach and maintain testability? Would it for example be wrong to pass the string directly to the IMapConverter and handle it there?