"415 Unsupported Media Type" for Content-Type "application/csp-report" in ASP.NET Core

The following example shows how to add support to the SystemTextJsonInputFormatter for handling additional media-types:

services.AddControllers(options =>
{
    var jsonInputFormatter = options.InputFormatters
        .OfType<SystemTextJsonInputFormatter>()
        .Single();

    jsonInputFormatter.SupportedMediaTypes.Add("application/csp-report");
});

This is a two-step process:

1. Interrogate the configured list of input-formatters to find the SystemTextJsonInputFormatter.
2. Add application/csp-report to its existing list of supported media-types (application/json, text/json, and application/*+json).

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If you're using Json.NET instead of System.Text.Json, the approach is similar:

services.AddControllers(options =>
{
    var jsonInputFormatter = options.InputFormatters
        .OfType<NewtonsoftJsonInputFormatter>()
        .First();

    jsonInputFormatter.SupportedMediaTypes.Add("application/csp-report");
})

There are two small differences:

1. The type is NewtonsoftJsonInputFormatter instead of SystemTextJsonInputFormatter.
2. There are two instances of this type in the collection, so we target the first (see this answer for the specifics).

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See Input Formatters in the ASP.NET Core docs to learn more about those.