title: Specify content-type for files in multipart/form-data for Swagger
body: <p>I have implemented endpoint with this signature</p>
  
  <pre><code>[HttpPost("Test")]
  public IActionResult MyTest([Required] IFormFile pdf, [Required] IFormFile image)
  {
      // some stuff...
  
      return Ok();
  }
  </code></pre>
  
  <p>this generates following entry in swagger.json <em>(the relevant part)</em></p>
  
  <pre><code>"content": {
      "multipart/form-data": {
          "schema": {
              "required": [
                  "image",
                  "pdf"
              ],
              "type": "object",
              "properties": {
                  "pdf": {
                      "type": "string",
                      "format": "binary"
                  },
                  "image": {
                      "type": "string",
                      "format": "binary"
                  }
              }
          },
          "encoding": {
              "pdf": {
                  "style": "form"
              },
              "image": {
                  "style": "form"
              }
          }
      }
  }
  </code></pre>
  
  <p>but, I also need specify encoding, like in <a href="https://swagger.io/docs/specification/describing-request-body/multipart-requests/#content-type" rel="noreferrer">the specs</a> (v3). So for my task, that JSON should look like this, I think...</p>
  
  <pre><code>"encoding": {
      "pdf": {
          "style": "form",
          "contentType": "application/pdf"
      },
      "image": {
          "style": "form",
          "contentType": "image/png, image/jpeg"
      }
  }
  </code></pre>
  
  <p><strong>But how can I do that from code?</strong> I thought about <a href="https://github.com/domaindrivendev/Swashbuckle.AspNetCore#enrich-parameter-metadata" rel="noreferrer">SwaggerParameter attribute</a>, but it contains only description and required flag...</p>
  
  <p>I'm using Swashbuckle.AspNetCore NuGeT package (version 5.0.0-rc2) on .NET Core 2.2.</p>