Generated OpenAPI docs overrides [Consumes] attribute through [FromForm]

[Zylvian]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When creating a form that is expecting an image to be uploaded via [Consumes("multipart/form-data")], the generated OpenAPI document will define the expected value as application/x-www-form-urlencoded.

Controller:

[HttpPost]
[Consumes("multipart/form-data")]
public async Task<ActionResult<SomeDto>> ImageToData(
        [FromForm] IFormFile image)
    {
    ...

Generated OpenAPI document:

...
"requestBody": {
          "content": {
            "application/x-www-form-urlencoded": {
              "schema": {
...

Expected Behavior

I expect the generated OpenAPI docs to correctly annotate the endpoint as a
"multipart/form-data"
endpoint, as opposed to an
"application/x-www-form-urlencoded"
endpoint.

Steps To Reproduce

1. Create an AspNetCore API with generated OpenAPI documentation.
2. On a controller, add an endpoint with the [Consumes] attribute and a [FromForm] parameter.
3. Browse the generated OpenAPI document (e.g http://localhost:5211/openapi/v1.json)
4. Note the discrepancy between the Consumes attribute and the generated documentation.

Exceptions (if any)

No response

.NET Version

9.0.200

Anything else?

ASP.NET Core: 9.0.2

dotnet --info

.NET SDK:
Version: 9.0.200
Commit: 90e8b202f2
Workload version: 9.0.200-manifests.b4a8049f
MSBuild version: 17.13.8+cbc39bea8

Runtime Environment:
OS Name: ubuntu
OS Version: 24.04
OS Platform: Linux
RID: linux-x64
Base Path: /home/a46039/.asdf/installs/dotnet/9.0.200/sdk/9.0.200/

.NET workloads installed:
There are no installed workloads to display.
Configured to use loose manifests when installing new manifests.

Host:
Version: 9.0.2
Architecture: x64
Commit: 80aa709f5d

.NET SDKs installed:
9.0.200 [/home/a46039/.asdf/installs/dotnet/9.0.200/sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 9.0.2 [/home/a46039/.asdf/installs/dotnet/9.0.200/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 9.0.2 [/home/a46039/.asdf/installs/dotnet/9.0.200/shared/Microsoft.NETCore.App]

Other architectures found:
None

Environment variables:
Not set

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download

[Zylvian]

I have implemented a temporary fix, for anbody facing the same issue:

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.OpenApi;
using Microsoft.OpenApi.Models;

namespace ApiService.Utility;

public class MultipartFormDataOperationTransformer : IOpenApiOperationTransformer
{
    public Task TransformAsync(OpenApiOperation operation, OpenApiOperationTransformerContext context,
        CancellationToken cancellationToken)
    {
        // Retrieve the ApiDescription for the current operation
        var apiDescription = context.Description;

        // Check if the endpoint has the Consumes attribute with "multipart/form-data"
        var consumesAttribute = apiDescription.ActionDescriptor.EndpointMetadata
            .OfType<ConsumesAttribute>()
            .FirstOrDefault(attr => attr.ContentTypes.Contains("multipart/form-data"));

        if (consumesAttribute != null)
        {
            // Identify keys that are incorrectly set to 'application/x-www-form-urlencoded'
            var keysToUpdate = operation.RequestBody?.Content.Keys
                .Where(key =>
                    key.Equals("application/x-www-form-urlencoded", System.StringComparison.OrdinalIgnoreCase))
                .ToList();

            // Update each identified key to 'multipart/form-data'
            if (keysToUpdate != null)
            {
                foreach (var key in keysToUpdate)
                {
                    var mediaType = operation.RequestBody.Content[key];
                    operation.RequestBody.Content.Remove(key);
                    operation.RequestBody.Content["multipart/form-data"] = mediaType;
                }
            }
        }

        return Task.CompletedTask;
    }
}             

Program.cs:

...
builder.Services.AddOpenApi(options =>
{
    options.AddOperationTransformer<MultipartFormDataOperationTransformer>();
});
...