Write JSON file in HTTPREQUEST

Hi Guys,

how can I write a json file in httprequest, I'm using the below but when I check what's in mystring it's actually truncated and doen't include all my file?

 Set stream=##class(%FileBinaryStream).%New()
    S FilePath="D:\Allied_InterfaceData\ResponseFiles\CollectionData\CollectionGood\CollectionGood_9431312.json"
   Set File = ##class(%File).%New(FilePath)
Do File.Open("R")
Set Line = File.Read(1000)
While (File.Read(1000)'="") { S mystring=mystring_File.Read(1000)
} 
  Set Httprequest=##class(%Net.HttpRequest).%New()
Set Httprequest.SSLConfiguration="RTLS"
Set Httprequest.Server="vibra-api-dev.azurewebsites.net"
Set Httprequest.Timeout=30
Set Httprequest.Https=1
set Httprequest.ContentType="application/json"
Do Httprequest.SetHeader("Accept","application/json")
Do Httprequest.SetHeader("Accept-Language","en_US")
    Do Httprequest.SetHeader("Authorization","Bearer "_^Token)
  D Httprequest.EntityBody.Write(mystring)
Set tSc=Httprequest.Post("/API/SENSORS/VIBRATION")
Set StateCode=Httprequest.HttpResponse.StatusCode
Set State=Httprequest.HttpResponse.StatusLine
Quit StateCode_"|"_State

Thanks