test: improve the test by verifying we got a mp4

SoundSwitch.Tests/FileDownloaderTests.cs

@@ -11,6 +11,20 @@ namespace SoundSwitch.Tests;
 [TestFixture]
 public class FileDownloaderTests
 {
+    private bool IsMP4(MemoryStream stream)
+    {
+        // Check if the stream has enough bytes for the magic number
+        if (stream.Length < 8)
+            return false;
+
+        // Read the first 8 bytes from the stream
+        var buffer = new byte[8];
+        stream.Position = 0;
+        stream.Read(buffer, 0, 8);
+
+        // Check if the byte sequence matches the MP4 magic number
+        return buffer[0] == 0x00 && buffer[1] == 0x00 && buffer[2] == 0x00 && buffer[3] == 0x14 && buffer[4] == 0x66 && buffer[5] == 0x74 && buffer[6] == 0x79 && buffer[7] == 0x70;
+    }
     [Test]
     public async Task DownloadTest()
     {
@@ -20,15 +34,16 @@ public async Task DownloadTest()
         var cancellationToken = default(CancellationToken);
         long downloaded = 0;
         long fileSize = 0;
-        
+
         // Act
         await FileDownloader.DownloadFileAsync(uri, stream, (l, l1) =>
         {
             downloaded = l;
             fileSize = l1;
-        } , cancellationToken);
-        
+        }, cancellationToken);
+
         //Assert
         downloaded.Should().Be(fileSize);
+        IsMP4(stream).Should().BeTrue();
     }
 }