Trim dots at the end of filenames on Windows

Closes #977

DiscordChatExporter.Core/Utils/PathEx.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.IO;
+using System.Runtime.InteropServices;
 using System.Text;
 
 namespace DiscordChatExporter.Core.Utils;
@@ -15,6 +16,14 @@ public static class PathEx
         foreach (var c in path)
             buffer.Append(!InvalidFileNameChars.Contains(c) ? c : '_');
 
+        // File names cannot end with a dot on Windows
+        // https://github.com/Tyrrrz/DiscordChatExporter/issues/977
+        if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+        {
+            while (buffer.Length > 0 && buffer[^1] == '.')
+                buffer.Remove(buffer.Length - 1, 1);
+        }
+
         return buffer.ToString();
     }