[core] Fix handling of binary data in core-client-rest (Azure#28054)

### Packages impacted by this PR

- `@azure-rest/core-client`

### Describe the problem that is addressed by this PR

- Remove conversion of Uint8Array to binary string with
`application/octet-stream` content type
- `core-rest-pipeline` handles Uint8Array as a request body type so this
conversion should not be necessary
- Remove conversion of Uint8Array to binary string in
`multipart/form-data` requests and convert to `Blob` instead
- With the new multipart work, support for `Blob` is much improved and
this workaround should no longer be required.

sdk/core/core-client-rest/CHANGELOG.md

@@ -8,6 +8,8 @@
 
 ### Bugs Fixed
 
+- Fix serialization of binary data in `multipart/form-data` requests and in binary request bodies.
+
 ### Other Changes
 
 - Upgrade dependency `@azure/abort-controller` to `^2.0.0`.

sdk/core/core-client-rest/src/sendRequest.ts

@@ -17,7 +17,6 @@ import {
 import { getCachedDefaultHttpsClient } from "./clientHelpers";
 import { isReadableStream } from "./helpers/isReadableStream";
 import { HttpResponse, RequestParameters } from "./common";
-import { binaryArrayToString } from "./helpers/getBinaryBody";
 
 /**
  * Helper function to send request used by the client
@@ -173,13 +172,7 @@ function getRequestBody(body?: unknown, contentType: string = ""): RequestBody {
   }
 
   if (ArrayBuffer.isView(body)) {
-    if (body instanceof Uint8Array) {
-      return firstType === "application/octet-stream"
-        ? { body }
-        : { body: binaryArrayToString(body) };
-    } else {
-      return { body: JSON.stringify(body) };
-    }
+    return { body: body instanceof Uint8Array ? body : JSON.stringify(body) };
   }
 
   switch (firstType) {
@@ -202,7 +195,7 @@ function isFormData(body: unknown): body is FormDataMap {
 }
 
 /**
- * Checks if binary data is in Uint8Array format, if so decode it to a binary string
+ * Checks if binary data is in Uint8Array format, if so wrap it in a Blob
  * to send over the wire
  */
 function processFormData(formData?: FormDataMap) {
@@ -215,7 +208,9 @@ function processFormData(formData?: FormDataMap) {
   for (const element in formData) {
     const item = formData[element];
     if (item instanceof Uint8Array) {
-      processedFormData[element] = binaryArrayToString(item);
+      // Some RLCs take a Uint8Array for the parameter, whereas FormDataMap expects
+      // a File or a Blob, so we need to wrap it.
+      processedFormData[element] = new Blob([item]);
     } else {
       processedFormData[element] = item;
     }

sdk/core/core-client-rest/test/sendRequest.spec.ts

@@ -18,9 +18,8 @@ describe("sendRequest", () => {
   describe("Binary content", () => {
     it("should handle request body as Uint8Array", async () => {
       const mockPipeline: Pipeline = createEmptyPipeline();
-      const expectedBody = "foo";
       mockPipeline.sendRequest = async (_client, request) => {
-        assert.equal(request.body, expectedBody);
+        assert.sameOrderedMembers([...(request.body as Uint8Array)], [...foo]);
         return { headers: createHttpHeaders() } as PipelineResponse;
       };
 
@@ -70,7 +69,12 @@ describe("sendRequest", () => {
       const expectedFormData = { fileName: "foo.txt", file: "foo" };
       const mockPipeline: Pipeline = createEmptyPipeline();
       mockPipeline.sendRequest = async (_client, request) => {
-        assert.deepEqual(request.formData, expectedFormData);
+        assert.equal(request.formData?.fileName, "foo.txt");
+        assert.instanceOf(request.formData?.file, Blob);
+        assert.sameOrderedMembers(
+          [...new Uint8Array(await (request.formData?.file as Blob).arrayBuffer())],
+          [...foo]
+        );
         return { headers: createHttpHeaders() } as PipelineResponse;
       };