Allow setting parameter content type

docs/docs/usage/request.md

@@ -71,11 +71,30 @@ request.AddParameter("name", "Væ üé", false); // don't encode the value
 If you have files, RestSharp will send a `multipart/form-data` request. Your parameters will be part of this request in the form:
 
 ```
+Content-Type: text/plain; charset=utf-8
 Content-Disposition: form-data; name="parameterName"
 
 ParameterValue
 ```
 
+Sometimes, you need to override the default content type for the parameter when making a multipart form call. It's possible to do by setting the `ContentType` property of the parameter object. As an example, the code below will create a POST parameter with JSON value, and set the appropriate content type:
+
+```csharp
+var parameter = new GetOrPostParameter("someJson", "{\"attributeFormat\":\"pdf\"}") {
+    ContentType = "application/json"
+};
+request.AddParameter(parameter);
+```
+
+When the request is set to use multipart content, the parameter will be sent as part of the request with the specified content type:
+
+```
+Content-Type: application/json; charset=utf-8
+Content-Disposition: form-data; name="someJson"
+
+{"attributeFormat":"pdf"}
+```
+
 You can also add `GetOrPost` parameter as a default parameter to the client. This will add the parameter to every request made by the client.
 
 ```csharp

src/RestSharp/Parameters/Parameter.cs

@@ -32,13 +32,29 @@ protected Parameter(string? name, object? value, ParameterType type, bool encode
     }
 
     /// <summary>
-    /// MIME content type of the parameter
+    /// Content type of the parameter. Normally applies to the body parameter, or POST parameter in multipart requests.
     /// </summary>
-    public ContentType ContentType { get; protected init; } = ContentType.Undefined;
-    public string?       Name   { get; }
-    public object?       Value  { get; }
-    public ParameterType Type   { get; }
-    public bool          Encode { get; }
+    public ContentType ContentType { get; set; } = ContentType.Undefined;
+
+    /// <summary>
+    /// Parameter name
+    /// </summary>
+    public string? Name { get; }
+
+    /// <summary>
+    /// Parameter value
+    /// </summary>
+    public object? Value { get; }
+
+    /// <summary>
+    /// Parameter type
+    /// </summary>
+    public ParameterType Type { get; }
+
+    /// <summary>
+    /// Indicates if the parameter value should be encoded or not.
+    /// </summary>
+    public bool Encode { get; }
 
     /// <summary>
     /// Return a human-readable representation of this parameter
@@ -48,6 +64,15 @@ protected Parameter(string? name, object? value, ParameterType type, bool encode
 
     protected virtual string ValueString => Value?.ToString() ?? "null";
 
+    /// <summary>
+    /// Creates a parameter object of based on the type
+    /// </summary>
+    /// <param name="name">Parameter name</param>
+    /// <param name="value">Parameter value</param>
+    /// <param name="type">Parameter type</param>
+    /// <param name="encode">Indicates if the parameter value should be encoded</param>
+    /// <returns></returns>
+    /// <exception cref="ArgumentOutOfRangeException"></exception>
     public static Parameter CreateParameter(string? name, object? value, ParameterType type, bool encode = true)
         // ReSharper disable once SwitchExpressionHandlesSomeKnownEnumValuesWithExceptionInDefault
         => type switch {

test/RestSharp.Tests.Integrated/MultipartFormDataTests.cs

@@ -13,6 +13,7 @@ public MultipartFormDataTests(ITestOutputHelper output) {
         _server = WireMockServer.Start();
 
         _capturer = _server.ConfigureBodyCapturer(Method.Post);
+
         var options = new RestClientOptions($"{_server.Url!}{RequestBodyCapturer.Resource}") {
             ConfigureMessageHandler = handler => new HttpTracerHandler(handler, new OutputLogger(output), HttpMessageParts.All)
         };
@@ -180,7 +181,7 @@ public async Task MultipartFormDataAsync() {
 
         _capturer.Body.Should().Be(expected);
     }
-    
+
     [Fact]
     public async Task MultipartFormData_Without_File_Creates_A_Valid_RequestBody() {
         using var client = new RestClient(_server.Url!);
@@ -206,4 +207,29 @@ public async Task MultipartFormData_Without_File_Creates_A_Valid_RequestBody() {
         var actual = capturer.Body!.Replace("\n", string.Empty).Split('\r');
         actual.Should().Contain(expectedBody);
     }
+
+    [Fact]
+    public async Task PostParameter_contentType_in_multipart_form() {
+        using var client = new RestClient(_server.Url!);
+
+        var request = new RestRequest(RequestBodyCapturer.Resource, Method.Post) {
+            AlwaysMultipartFormData = true
+        };
+        var capturer = _server.ConfigureBodyCapturer(Method.Post);
+
+        const string parameterName = "Arequest";
+        const string parameterValue = "{\"attributeFormat\":\"pdf\"}";
+
+        var parameter = new GetOrPostParameter(parameterName, parameterValue) {
+            ContentType = "application/json"
+        };
+        request.AddParameter(parameter);
+
+        await client.ExecuteAsync(request);
+
+        var actual = capturer.Body!.Replace("\n", string.Empty).Split('\r');
+        actual[1].Should().Be("Content-Type: application/json; charset=utf-8");
+        actual[2].Should().Be($"Content-Disposition: form-data; name={parameterName}");
+        actual[4].Should().Be(parameterValue);
+    }
 }