Hotfix: Multipart bytes[] part

common/changes/@typespec/openapi3/hotfix-multipart-bytes-array_2023-12-13-22-16.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@typespec/openapi3",
+      "comment": "Fix: Multipart part of type `bytes[]` is now treated as multiple binary part",
+      "type": "none"
+    }
+  ],
+  "packageName": "@typespec/openapi3"
+}

packages/openapi3/src/schema-emitter.ts

@@ -305,17 +305,27 @@ export class OpenAPI3SchemaEmitter extends TypeEmitter<
     return props;
   }
 
+  #isBytesKeptRaw(type: Type) {
+    const program = this.emitter.getProgram();
+    return (
+      type.kind === "Scalar" && type.name === "bytes" && getEncode(program, type) === undefined
+    );
+  }
+
   modelPropertyLiteral(prop: ModelProperty): EmitterOutput<object> {
     const program = this.emitter.getProgram();
     const isMultipart = this.#getContentType().startsWith("multipart/");
-    if (
-      isMultipart &&
-      prop.type.kind === "Scalar" &&
-      prop.type.name === "bytes" &&
-      getEncode(program, prop.type) === undefined &&
-      getEncode(program, prop) === undefined
-    ) {
-      return { type: "string", format: "binary" };
+    if (isMultipart) {
+      if (this.#isBytesKeptRaw(prop.type) && getEncode(program, prop) === undefined) {
+        return { type: "string", format: "binary" };
+      }
+      if (
+        prop.type.kind === "Model" &&
+        isArrayModelType(program, prop.type) &&
+        this.#isBytesKeptRaw(prop.type.indexer.value)
+      ) {
+        return { type: "array", items: { type: "string", format: "binary" } };
+      }
     }
 
     const refSchema = this.emitter.emitTypeReference(prop.type, {

packages/openapi3/test/multipart.test.ts

@@ -38,6 +38,27 @@ describe("typespec-autorest: multipart", () => {
     });
   });
 
+  it("part of type `bytes[]` produce `type: array, items: {type: string, format: binary}`", async () => {
+    const res = await openApiFor(
+      `
+      op upload(@header contentType: "multipart/form-data", profileImage: bytes[]): void;
+      `
+    );
+    const op = res.paths["/"].post;
+    deepStrictEqual(op.requestBody.content["multipart/form-data"], {
+      schema: {
+        type: "object",
+        properties: {
+          profileImage: {
+            type: "array",
+            items: { type: "string", format: "binary" },
+          },
+        },
+        required: ["profileImage"],
+      },
+    });
+  });
+
   it("part of type `string` produce `type: string`", async () => {
     const res = await openApiFor(
       `