Fix for extremely long file extensions (#812)

DiscordChatExporter.Core/Exporting/MediaDownloader.cs

@@ -104,6 +104,14 @@ internal partial class MediaDownloader
         var fileNameWithoutExtension = Path.GetFileNameWithoutExtension(fileName);
         var fileExtension = Path.GetExtension(fileName);
         
+        // Probably not a file extension, just a dot in a long file name
+        // https://github.com/Tyrrrz/DiscordChatExporter/issues/708
+        if (fileExtension.Length > 41)
+        {
+            fileNameWithoutExtension = fileName;
+            fileExtension = "";
+        }
+
         return PathEx.EscapeFileName(fileNameWithoutExtension.Truncate(42) + '-' + urlHash + fileExtension);
     }
 }