Use input file OpenApi format as the default output format

Author: danielmbaluka

src/Microsoft.OpenApi.Hidi/OpenApiService.cs

@@ -22,7 +22,7 @@ public static void ProcessOpenApiDocument(
             string input,
             FileInfo output,
             OpenApiSpecVersion version,
-            OpenApiFormat format,
+            OpenApiFormat? format,
             string filterByOperationIds,
             string filterByTags,
             bool inline,
@@ -91,7 +91,9 @@ public static void ProcessOpenApiDocument(
             {
                 ReferenceInline = inline ? ReferenceInlineSetting.InlineLocalReferences : ReferenceInlineSetting.DoNotInlineReferences
             };
-            IOpenApiWriter writer = format switch
+
+            var openApiFormat = format ?? GetOpenApiFormat(input);
+            IOpenApiWriter writer = openApiFormat switch
             {
                 OpenApiFormat.Json => new OpenApiJsonWriter(textWriter, settings),
                 OpenApiFormat.Yaml => new OpenApiYamlWriter(textWriter, settings),
@@ -156,5 +158,17 @@ internal static void ValidateOpenApiDocument(string input)
 
             Console.WriteLine(statsVisitor.GetStatisticsReport());
         }
+
+        private static OpenApiFormat GetOpenApiFormat(string input)
+        {
+            if (!input.StartsWith("http") && Path.GetExtension(input) == ".json")
+            {
+                return OpenApiFormat.Json;
+            }
+            else
+            {
+                return OpenApiFormat.Yaml;
+            }
+        }
     }
 }

src/Microsoft.OpenApi.Hidi/Program.cs

@@ -32,7 +32,7 @@ static async Task<int> Main(string[] args)
                 new Option("--filterByOperationIds", "Filters OpenApiDocument by OperationId(s) provided", typeof(string)),
                 new Option("--filterByTags", "Filters OpenApiDocument by Tag(s) provided", typeof(string))
             };
-            transformCommand.Handler = CommandHandler.Create<string, FileInfo, OpenApiSpecVersion, OpenApiFormat, string, string, bool, bool>(
+            transformCommand.Handler = CommandHandler.Create<string, FileInfo, OpenApiSpecVersion, OpenApiFormat?, string, string, bool, bool>(
                 OpenApiService.ProcessOpenApiDocument);
 
             rootCommand.Add(transformCommand);