Encode screenshot as JPEG when it has the extension (#2982)

src/BizHawk.Client.EmuHawk/MainForm.cs

@@ -1193,8 +1193,15 @@ namespace BizHawk.Client.EmuHawk
 			using (var bb = Config.ScreenshotCaptureOsd ? CaptureOSD() : MakeScreenshotImage())
 			{
 				using var img = bb.ToSysdrawingBitmap();
+				if (Path.GetExtension(path).ToUpper() == ".JPG")
+				{
+					img.Save(fi.FullName, ImageFormat.Jpeg);
+				}
+				else
+				{
 					img.Save(fi.FullName, ImageFormat.Png);
 				}
+			}
 
 			AddOnScreenMessage($"{fi.Name} saved.");
 		}