Add mystery gift file drag&drop to slot

pgt was a pkm size, check all sizes

MainWindow/Main.cs

@@ -3344,7 +3344,7 @@ private void pbBoxSlot_DragDrop(object sender, DragEventArgs e)
                     return;
                 string file = files[0];
                 FileInfo fi = new FileInfo(file);
-                if (!PKX.getIsPKM(fi.Length))
+                if (!PKX.getIsPKM(fi.Length) && !MysteryGift.getIsMysteryGift(fi.Length))
                 { openQuick(file); return; }
 
                 byte[] data = File.ReadAllBytes(file);

MysteryGifts/MysteryGift.cs

@@ -5,6 +5,10 @@ namespace PKHeX
 {
     public abstract class MysteryGift
     {
+        internal static bool getIsMysteryGift(long len)
+        {
+            return new[] { WC6.SizeFull, WC6.Size, PGF.Size, PGT.Size, PCD.Size }.Contains((int)len);
+        }
         internal static MysteryGift getMysteryGift(byte[] data, string ext)
         {
             if (data.Length == WC6.SizeFull && ext == ".wc6full")