Allow reading matroska files with "mp4" ext

Related to #9010

src/libse/Common/FileUtil.cs

@@ -859,6 +859,18 @@ namespace Nikse.SubtitleEdit.Core.Common
             }
         }
 
+        public static bool IsMatroskaFileFast(string fileName)
+        {
+            using (var fs = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
+            {
+                var buffer = new byte[4];
+                fs.Read(buffer, 0, buffer.Length);
+
+                // 1a 45 df a3
+                return buffer[0] == 0x1a && buffer[1] == 0x45 && buffer[2] == 0xdf && buffer[3] == 0xa3;
+            }
+        }
+
         /// <summary>
         /// Checks if a file is locked by attempting to open it with exclusive read access.
         /// </summary>

src/ui/Forms/Main.cs

@@ -2952,7 +2952,12 @@ namespace Nikse.SubtitleEdit.Forms
                 }
             }
 
-            if (ext == ".mkv" || ext == ".mks")
+            if (ext == ".mkv" || ext == ".mks" ||
+
+                // allow for mkv files with wrong extension
+                ((ext == ".mp4" || ext == ".mov" || ext == ".m4v" || ext == ".wmv") && FileUtil.IsMatroskaFileFast(fileName) && FileUtil.IsMatroskaFile(fileName))
+                
+                ) 
             {
                 ImportSubtitleFromMatroskaFile(fileName);
                 return;