checking base64 format fix

WebAPI/Controllers/DataInboxController.cs

@@ -137,7 +137,26 @@ public class DataInboxController : Controller
     // helpers
     private bool IsBase64String(string data)
     {
-        var bytes = new Span<byte>(new byte[256]); 
-        return Convert.TryFromBase64String(data, bytes, out _);
+        if (string.IsNullOrEmpty(data))
+        {
+            return false;
+        }
+        try
+        {
+            var base64Bytes = Convert.FromBase64String(data);
+            
+            var utf8String = Encoding.UTF8.GetString(base64Bytes);
+            
+            var reEncoded = Convert.ToBase64String(Encoding.UTF8.GetBytes(utf8String));
+            return data.TrimEnd('=') == reEncoded.TrimEnd('=');
+        }
+        catch (FormatException)
+        {
+            return false;
+        }
+        catch (DecoderFallbackException)
+        {
+            return false;
+        }
     }
 }