Use MediaTypeNames.

Author: sarahelsaig

Directory.Packages.props

@@ -6,7 +6,7 @@
     <!-- The Orchard Core version should always be x.y.0 of the latest minor version for maximum compatibility when
          distributed as NuGet packages. On the other hand, the consuming projects (including OrchardCore.Commerce.Web)
          should use Orchard Core references for the latest patch version to pull all versions up in the final app. -->
-    <OrchardCoreVersion>3.0.0-preview-18937</OrchardCoreVersion>
+    <OrchardCoreVersion>3.0.0-preview-18941</OrchardCoreVersion>
   </PropertyGroup>
 
   <ItemGroup>

src/Modules/OrchardCore.Commerce.Payment.Exactly/Services/ExactlyApiHandler.cs

@@ -1,5 +1,6 @@
 using Microsoft.Extensions.Options;
 using OrchardCore.Commerce.Payment.Exactly.Models;
+using OrchardCore.Infrastructure;
 using System;
 using System.Net.Http;
 using System.Net.Http.Headers;
@@ -25,7 +26,7 @@ protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage reques
         const BindingFlags privateFieldFlags = BindingFlags.Instance | BindingFlags.NonPublic;
         var allowedHeaderTypes = typeof(HttpHeaders).GetField("_allowedHeaderTypes", privateFieldFlags);
         allowedHeaderTypes!.SetValue(request.Headers, Enum.Parse(allowedHeaderTypes.FieldType, "All"));
-        request.Headers.Add("Content-Type", "application/vnd.api+json");
+        request.Headers.Add("Content-Type", MediaTypeNamesExtended.Application.JsonVendeorPrefix);
 #pragma warning restore S3011
 
         return base.SendAsync(request, cancellationToken);