Firebase Storage: An unknown error occurred, please check the error payload for server response. (storage/unknown)

### Operating System

- macOS 10.15.7 (Catalina) — majority of occurrences - Windows 10 — at least one confirmed occurrence (Chrome 129.0.0.0)

### Environment (if applicable)

Browser: Google Chrome, Safari

### Firebase SDK Version

12.4.0

### Firebase SDK Product(s)

Storage

### Project Tooling

Framework: Angular (v22.0.1)
Language: TypeScript (v6.0.3)
Firebase SDK: firebase (v12.14.0)
Node.js (v24.16.0)

### Detailed Problem Description

We are trying to upload audio files (MP3, up to 15 MB — this is our app's enforced maximum) to Firebase Storage using `uploadBytesResumable`. The upload succeeds the vast majority of the time. However, intermittently, the upload fails with:

Firebase Storage: An unknown error occurred, please check the error payload for server response. (storage/unknown)

The full error object captured from our analytics event log is:

```
{
"error": {
"_baseMessage": "Firebase Storage: An unknown error occurred, please check the error payload for server response. (storage/unknown)",
"code": "storage/unknown",
"customData": {
"serverResponse": ""
},
"name": "FirebaseError",
"status_": 400
},
"path": "sheet_music/92oBKIzIMSw8STjnndjL",
"uploadType": "blob_resumable"
}
```

The screenshot below shows the event logs for the same session. The start upload event fires at 12:04:10, and upload failed fires at 12:04:15 — approximately 5 seconds later.

### Steps and code to reproduce issue

We have not been able to create a minimal reproduction, as this error only appears intermittently (flaky) across real user sessions.

However, the error consistently appears in one of two scenarios in our app:

1. Direct audio upload — A user selects an MP3 file (up to 15 MB) via a file picker and the app calls uploadBytesResumable to store it at a sheet_music/{documentId} path. Most uploads succeed; occasionally the task fails immediately with storage/unknown and an empty serverResponse.
2. Copy on submit — When a user submits a form, the app fetches the previously uploaded audio blob from Storage (getBlob) and immediately re-uploads it to a new sheet_music/{submittedPath} path using uploadBytesResumable. The error also surfaces here intermittently.

In both cases, the upload fails with serverResponse: "" and status_: 400. We cannot determine from the available data at which phase of the resumable upload the failure occurs.