Fix incorrect OpenAPI definition when using FromForm with IEnumerable

src/OpenApi/src/Services/OpenApiDocumentService.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Collections;
 using System.Collections.Concurrent;
 using System.Collections.Frozen;
 using System.ComponentModel;
@@ -665,7 +666,11 @@ private async Task<OpenApiRequestBody> GetFormRequestBody(
                     }
                     else
                     {
-                        if (isComplexType)
+                        // Identify if this is a collection (excluding string)
+                        var isCollection = typeof(IEnumerable).IsAssignableFrom(description.Type) && description.Type != typeof(string);
+
+                        // Only allow non-collection complex types (POCOs) to take over the root body
+                        if (isComplexType && !isCollection)
                         {
                             complexTypeSchema = parameterSchema;
                         }

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiDocumentService/OpenApiDocumentServiceTests.RequestBody.cs

@@ -1076,6 +1076,34 @@ await VerifyOpenApiDocument(action, document =>
         });
     }
 
+    [Fact]
+    public async Task GetOpenApiRequestBody_HandlesFromFormWithIEnumerable()
+    {
+        // Arrange
+        var builder = CreateBuilder();
+
+        // Act
+        builder.MapPost("/test", ([FromForm] IEnumerable<int> values) => { });
+
+        // Assert
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            var paths = Assert.Single(document.Paths.Values);
+            var operation = paths.Operations[HttpMethod.Post];
+            Assert.NotNull(operation.RequestBody);
+            var content = operation.RequestBody.Content;
+            Assert.Contains("multipart/form-data", content.Keys);
+            var formSchema = content["multipart/form-data"].Schema;
+
+            Assert.Equal(JsonSchemaType.Object, formSchema.Type);
+            Assert.NotNull(formSchema.Properties);
+            Assert.Contains("values", formSchema.Properties);
+            var valuesProperty = formSchema.Properties["values"];
+            Assert.Equal(JsonSchemaType.Array, valuesProperty.Type);
+            Assert.Equal(JsonSchemaType.Integer, valuesProperty.Items.Type);
+        });
+    }
+
     [Route("/form-mixed-types")]
     private void ActionWithMixedFormTypes([FromForm] Todo todo, IFormFile formFile, [FromForm] Guid guid) { }