diff --git a/packages/@uppy/aws-s3-multipart/src/MultipartUploader.js b/packages/@uppy/aws-s3-multipart/src/MultipartUploader.js
index 68ca7fc714..b22728d1f2 100644
--- a/packages/@uppy/aws-s3-multipart/src/MultipartUploader.js
+++ b/packages/@uppy/aws-s3-multipart/src/MultipartUploader.js
@@ -26,23 +26,41 @@ function ensureInt (value) {
 
 export const pausingUploadReason = Symbol('pausing upload, not an actual error')
 
+/**
+ * A MultipartUploader instance is used per file upload to determine whether a
+ * upload should be done as multipart or as a regular S3 upload
+ * (based on the user-provided `shouldUseMultipart` option value) and to manage
+ * the chunk splitting.
+ */
 class MultipartUploader {
   #abortController = new AbortController()
 
+  /** @type {import("../types/chunk").Chunk[]} */
   #chunks
 
+  /** @type {{ uploaded: number, etag?: string, done?: boolean }[]} */
   #chunkState
 
+  /**
+   * The (un-chunked) data to upload.
+   *
+   * @type {Blob}
+   */
   #data
 
+  /** @type {import("@uppy/core").UppyFile} */
   #file
 
-  #uploadPromise
+  /** @type {boolean} */
+  #uploadHasStarted = false
 
+  /** @type {(err?: Error | any) => void} */
   #onError
 
+  /** @type {() => void} */
   #onSuccess
 
+  /** @type {typeof import('../types/index').AwsS3MultipartOptions["shouldUseMultipart"]} */
   #shouldUseMultipart
 
   #onReject = (err) => (err?.cause === pausingUploadReason ? null : this.#onError(err))
@@ -119,13 +137,14 @@ class MultipartUploader {
   }
 
   #createUpload () {
-    this.#uploadPromise = this
+    this
       .options.companionComm.uploadFile(this.#file, this.#chunks, this.#abortController.signal)
       .then(this.#onSuccess, this.#onReject)
+    this.#uploadHasStarted = true
   }
 
   #resumeUpload () {
-    this.#uploadPromise = this
+    this
       .options.companionComm.resumeUploadFile(this.#file, this.#chunks, this.#abortController.signal)
       .then(this.#onSuccess, this.#onReject)
   }
@@ -158,7 +177,7 @@ class MultipartUploader {
   }
 
   start () {
-    if (this.#uploadPromise) {
+    if (this.#uploadHasStarted) {
       if (!this.#abortController.signal.aborted) this.#abortController.abort(pausingUploadReason)
       this.#abortController = new AbortController()
       this.#resumeUpload()
diff --git a/packages/@uppy/aws-s3-multipart/src/index.js b/packages/@uppy/aws-s3-multipart/src/index.js
index 44d762f37e..6dc5719df1 100644
--- a/packages/@uppy/aws-s3-multipart/src/index.js
+++ b/packages/@uppy/aws-s3-multipart/src/index.js
@@ -239,6 +239,12 @@ class HTTPCommunicationQueue {
     }).abortOn(signal)
   }
 
+  /**
+   * @param {import("@uppy/core").UppyFile} file
+   * @param {import("../types/chunk").Chunk[]} chunks
+   * @param {AbortSignal} signal
+   * @returns {Promise<void>}
+   */
   async uploadFile (file, chunks, signal) {
     throwIfAborted(signal)
     if (chunks.length === 1 && !chunks[0].shouldUseMultipart) {
@@ -281,6 +287,14 @@ class HTTPCommunicationQueue {
     return this.#sendCompletionRequest(file, { key, uploadId, parts, signal }).abortOn(signal)
   }
 
+  /**
+   *
+   * @param {import("@uppy/core").UppyFile} file
+   * @param {number} partNumber
+   * @param {import("../types/chunk").Chunk} chunk
+   * @param {AbortSignal} signal
+   * @returns {Promise<object>}
+   */
   async uploadChunk (file, partNumber, chunk, signal) {
     throwIfAborted(signal)
     const { uploadId, key } = await this.getUploadId(file, signal)
diff --git a/packages/@uppy/aws-s3-multipart/types/chunk.d.ts b/packages/@uppy/aws-s3-multipart/types/chunk.d.ts
new file mode 100644
index 0000000000..cde2ffc351
--- /dev/null
+++ b/packages/@uppy/aws-s3-multipart/types/chunk.d.ts
@@ -0,0 +1,6 @@
+export interface Chunk {
+    getData: () => Blob
+    onProgress: (ev: ProgressEvent) => void
+    onComplete: (etag: string) => void
+    shouldUseMultipart: boolean
+}