allow both image-idx and image.idx in mnist

mindspore/ccsrc/minddata/dataset/engine/datasetops/source/mnist_op.cc

@@ -362,8 +362,8 @@ Status MnistOp::ParseMnistData() {
 }
 
 Status MnistOp::WalkAllFiles() {
-  const std::string kImageExtension = "idx3-ubyte";
-  const std::string kLabelExtension = "idx1-ubyte";
+  const std::string img_ext = "idx3-ubyte";
+  const std::string lbl_ext = "idx1-ubyte";
   const std::string train_prefix = "train";
   const std::string test_prefix = "t10k";
 
@@ -374,13 +374,13 @@ Status MnistOp::WalkAllFiles() {
   if (dir_it != nullptr) {
     while (dir_it->hasNext()) {
       Path file = dir_it->next();
-      std::string filename = file.Basename();
-      if (filename.find(prefix + "-images-" + kImageExtension) != std::string::npos) {
+      std::string fname = file.Basename();  // name of the mnist file
+      if ((fname.find(prefix + "-images") != std::string::npos) && (fname.find(img_ext) != std::string::npos)) {
         image_names_.push_back(file.toString());
-        MS_LOG(INFO) << "Mnist operator found image file at " << filename << ".";
-      } else if (filename.find(prefix + "-labels-" + kLabelExtension) != std::string::npos) {
+        MS_LOG(INFO) << "Mnist operator found image file at " << fname << ".";
+      } else if ((fname.find(prefix + "-labels") != std::string::npos) && (fname.find(lbl_ext) != std::string::npos)) {
         label_names_.push_back(file.toString());
-        MS_LOG(INFO) << "Mnist Operator found label file at " << filename << ".";
+        MS_LOG(INFO) << "Mnist Operator found label file at " << fname << ".";
       }
     }
   } else {