The value cannot be null or empty. (Parameter 'mediaType')

[guyulmaz]

DO NOT USE ISSUES FOR QUESTIONS

Describe the bug
A clear and concise description of what the bug is.
Hint: use a tool like https://requestbin.com to compare working and non-working requests.

.net 7 is giving this error. .net 6 dont have this problem.

var client = new RestClient();
var request = new RestRequest("...api", Method.Post);
request.AlwaysMultipartFormData = true;
request.AddParameter("x", "y");
request.AddParameter("xx", "yy");...
return await client.ExecuteAsync(request);

System.AggregateException: One or more errors occurred. (The value cannot be null or empty. (Parameter 'mediaType')) ---> System.ArgumentException: The value cannot be null or empty. (Parameter 'mediaType')
at System.Net.Http.Headers.MediaTypeHeaderValue.CheckMediaTypeFormat(String mediaType, String parameterName)
at System.Net.Http.StringContent..ctor(String content, Encoding encoding, String mediaType)
at RestSharp.RequestContent.AddPostParameters(ParametersCollection postParameters)
at RestSharp.RequestContent.BuildContent()
at RestSharp.RestClient.ExecuteInternal(RestRequest request, CancellationToken cancellationToken)
at RestSharp.RestClient.ExecuteAsync(RestRequest request, CancellationToken cancellationToken)

To Reproduce
Steps to reproduce the behavior, preferably using a code snippet.
Post the non-working request here as well if you made it work using Postman, Swagger, or any other client.

Expected behavior
A clear and concise description of what you expected to happen.
Post the working request here as well if you made it work using Postman, Swagger, or any other client.
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Stack trace
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Desktop (please complete the following information):

• OS: [e.g. macOS]
• .NET version [e.g. .NET 5]
• Version [e.g. 107.0.4]
  .net 7.0.2
  windows 10
  restsharp 108.0.3

Additional context
Add any other context about the problem here.

[alexeyzimarev]

Duplicate of #1969