Just a minimum example for anyone else struggling to upload an image with RestSharp from uwp app
using RestSharp;
private readonly RestClient Client;
public RestService() {
Client = new RestClient("http://localhost:1337/api");
}
public async Task UploadFileAsync(StorageFile file) {
var req = new RestRequest("/upload");
req.AddHeader("Content-Type", "multipart/form-data");
req.AddFile("files",file.Path);
await Client.PostAsync(req);
}