Integration tests fails on the Magic Bytes check on an upload-image endpoint

We have a simple rest API. One of its endpoints is to include magicBytes validation, which currently looks like this

private static bool HaveValidMagicBytes(IFormFile file)
{
    if (file == null || !MagicBytes.TryGetValue(file.ContentType, out byte[] magicBytes))
    {
        return false;
    }

    using (Stream stream = file.OpenReadStream())
    {
        var fileMagicBytes = new byte[magicBytes.Length];
        stream.Read(fileMagicBytes, 0, magicBytes.Length);
        bool haveValidMagicBytes = magicBytes.SequenceEqual(fileMagicBytes);
        return haveValidMagicBytes;
    }
}

We have existing integration tests for this, but I can't seem to get them to work with the above piece of code.

I currently prep the 'mocked image for the api endpoint':

private MultipartFormDataContent SetupFileImageTests(string fileName)
{
    var realFile = File.OpenRead(ImageHelper.GetImagePath()); //points to a local, valid file: TestData/test_image.PNG

    var mockedFile = new Mock<IFormFile>();
    var ms = new MemoryStream();
    var writer = new StreamWriter(ms);
    writer.Write(realFile);
    writer.Flush();
    ms.Position = 0;
    mockedFile.Setup(c => c.OpenReadStream()).Returns(ms);
    mockedFile.Setup(f => f.FileName).Returns(fileName).Verifiable();
    mockedFile.Setup(_ => _.CopyToAsync(It.IsAny<Stream>(), It.IsAny<CancellationToken>()))
        .Returns((Stream stream, CancellationToken token) => ms.CopyToAsync(stream))
        .Verifiable();

    writer.Write(FileContent);
    writer.Flush();
    ms.Position = 0;

    mockedFile.Setup(f => f.OpenReadStream()).Returns(ms);
    mockedFile.Setup(f => f.Length).Returns(ms.Length);
    mockedFile.Setup(f => f.FileName).Returns(fileName);

    var fileContent = new StreamContent(mockedFile.Object.OpenReadStream())
    {
        Headers =
        {
            ContentLength = mockedFile.Object.Length,
            ContentType = new MediaTypeHeaderValue("image/jpeg")
        }
    };

    return new MultipartFormDataContent
    {
        {fileContent, "file", fileName}
    };
}

I've tried adding an actual, real image, so that the check should succeed in theory, but it's still not. I'm not quite sure how to proceed.
haveValidMagicBytes is still false and thus causing the whole thing to crash.

My guess is that you use

new MediaTypeHeaderValue("image/jpeg")

but your file is

TestData/test_image.PNG

So you should either use image/png or change your test to ignore the content type, and instead compare the magic bytes with a list of known file formats you accept.