Returning Binary Data from ASP .Net Core MVC using Serverless and AWS API Gateway

I have a controller in ASP .Net MVC (dotnet core 2.1) and I'm trying to return a zip file to the user.

[HttpGet]
public async Task<ActionResult> GetLogFiles([FromQuery] string date, 
                                            [FromQuery] string startTime, 
                                            [FromQuery] string endTime)
{
    var decryptedFiles = new Dictionary<string, string>();

    // Code to get log files, decrypt them and add to dictionary

    using (var memoryStream = new MemoryStream())
    {
        using (var archive = new ZipArchive(memoryStream, ZipArchiveMode.Create, true))
        {
            foreach (var (name, data) in decryptedFiles)
            {
                _logger.LogInformation($"Adding {name}");

                var entry = archive.CreateEntry(name);

                using (var entryStream = entry.Open())
                {
                    using (var writer = new StreamWriter(entryStream, Encoding.GetEncoding("Windows-1252")))
                    {
                        writer.Write(data);
                    }
                }
            }
        }

        memoryStream.Seek(0, SeekOrigin.Begin);

        var rawData = memoryStream.ReadAll();

        var response = File(rawData, "application/zip", "logs.zip");

        return response;
    }
}

public static byte[] ReadAll(this Stream stream)
{
    var count = (int) stream.Length;

    var buffer = new byte[count];

    var offset = 0;

    while (offset < count)
    {
        var read = stream.Read(buffer, offset, count - offset);

        if (read == 0)
        {
            throw new EndOfStreamException();
        }

        offset += read;
    }

    return buffer;
}

When I visit the URI in Chrome, it seemingly downloads the zip file however, when I try to open it, Windows claims the file is invalid.

If I make the request in Fiddler, I can see that the server returns a blob of base 64. If I manually decode this base 64 and save the resulting bytes, it results in a valid zip file.

Here are all of the response headers (some redacted for reasons).

HTTP/1.1 200 OK
Content-Type: application/octet-stream
Content-Length: 508
Connection: keep-alive
Date: Tue, 17 Sep 2019 14:04:43 GMT
x-amzn-RequestId: [REDACTED]
Content-Disposition: attachment; filename=logs.zip; filename*=UTF-8''logs.zip
Strict-Transport-Security: max-age=2592000
x-amzn-Remapped-Content-Length: 381
x-amz-apigw-id: [REDACTED]
X-Amzn-Trace-Id: [REDACTED]
X-Cache: Miss from cloudfront
Via: 1.1 [REDACTED]
X-Amz-Cf-Pop: [REDACTED]
X-Amz-Cf-Id: [REDACTED]

Can anyone suggest what I might be doing wrong?

Any advice much appreciated.

Thanks,

Steve.