test.cs

static List<ImageData> GetImageBase64Strings(string html)
{
	var imageDatas = new List<ImageData>();
	var regex = new Regex("<img[^>]+?src=[\"']data:image/(?<format>[^;]+);base64,(?<data>[^\"']+)[\"'][^>]*>", RegexOptions.IgnoreCase);

	foreach (Match match in regex.Matches(html))
	{
		var format = match.Groups["format"].Value;
		var base64Content = match.Groups["data"].Value;
		var extension = GetFileExtension(format);

		imageDatas.Add(new ImageData
		{
			Base64Content = base64Content,
			Format = format,
			Extension = extension
		});
	}

	return imageDatas;
}

static string GetFileExtension(string format)
{
	return format.ToLower() switch
	{
		"jpeg" => ".jpg",
		"jpg" => ".jpg",
		"png" => ".png",
		"gif" => ".gif",
		"bmp" => ".bmp",
		"webp" => ".webp",
		"svg+xml" => ".svg",
		"tiff" => ".tif",
		"x-icon" => ".ico",
		_ => $".{format}"
	};
}

public class ImageData
{
	public string Base64Content { get; set; }
	public string Format { get; set; }
	public string Extension { get; set; }
}