Set optional enableRangeProcessing to true, you don't have to handle range header yourself.

public virtual FileStreamResult File(Stream fileStream, string contentType, bool enableRangeProcessing)

https://learn.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.mvc.fileresult.enablerangeprocessing?view=aspnetcore-9.0

Further reading source code https://github.com/dotnet/aspnetcore/blob/main/src/Shared/ResultsHelpers/FileResultHelper.cs#L90