@uppy/aws-s3: resolve all headers on response

[Murderlon]

Closes #5138

[github-actions]

Diff output files

diff --git a/packages/@uppy/aws-s3-multipart/lib/index.js b/packages/@uppy/aws-s3-multipart/lib/index.js
index aef977f..3b48db2 100644
--- a/packages/@uppy/aws-s3-multipart/lib/index.js
+++ b/packages/@uppy/aws-s3-multipart/lib/index.js
@@ -489,7 +489,7 @@ export default class AwsS3Multipart extends BasePlugin {
         };
         reject(error);
       });
-      xhr.addEventListener("load", ev => {
+      xhr.addEventListener("load", () => {
         cleanup();
         if (xhr.status === 403 && xhr.responseText.includes("<Message>Request has expired</Message>")) {
           const error = new Error("Request has expired");
@@ -507,8 +507,20 @@ export default class AwsS3Multipart extends BasePlugin {
           loaded: size,
           lengthComputable: true,
         });
-        const etag = xhr.getResponseHeader("ETag");
-        const location = xhr.getResponseHeader("Location");
+        const arr = xhr.getAllResponseHeaders().trim().split(/[\r\n]+/);
+        const headersMap = {
+          __proto__: null,
+        };
+        for (const line of arr) {
+          const parts = line.split(": ");
+          const header = parts.shift();
+          const value = parts.join(": ");
+          headersMap[header] = value;
+        }
+        const {
+          etag,
+          location,
+        } = headersMap;
         if (method.toUpperCase() === "POST" && location === null) {
           console.warn(
             "AwsS3/Multipart: Could not read the Location header. This likely means CORS is not configured correctly on the S3 Bucket. See https://uppy.io/docs/aws-s3-multipart#S3-Bucket-Configuration for instructions.",
@@ -524,12 +536,8 @@ export default class AwsS3Multipart extends BasePlugin {
         }
         onComplete == null || onComplete(etag);
         resolve({
+          ...headersMap,
           ETag: etag,
-          ...(location
-            ? {
-              location,
-            }
-            : undefined),
         });
       });
       xhr.addEventListener("error", ev => {