A possible reason for the issue observed could be an issue with serialization. What you can try is specifying the serialization settings for the returned FileResult:
public IHttpActionResult ChunkSave()
//...var settings = GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings;settings.ContractResolver = new CamelCasePropertyNamesContractResolver();return Json(fileBlob,settings);
Give this suggestion a try and let me know if it helps resolve the issue.
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