File attachment sync fails with EncryptedBlockstore errors in v0.23.0

## Bug Report: Screenshot File Access Fails for Synced Data with EncryptedBlockstore Errors

### Summary
File attachments (screenshots) work for local data but fail to load for synced data with `EncryptedBlockstore` errors and "car not ready" messages when using `toCloud()` attachment.

### Environment
- **Fireproof Version**: v0.23.0 (UPGRADE BRANCH)
- **Application**: Vibes DIY catalog system
- **Browser**: Chrome
- **Sync**: Using `toCloud()` attachment for authenticated users

### Symptoms

#### Working Case (Local Data)
- **Vibe ID**: `fmehjlyodri54k8rfr`
- **Result**: ✅ `Screenshot size: 34731 bytes`
- **Display**: File attachment renders correctly in UI
- **Database**: Local-only (`userId = 'local'`, no toCloud attachment)

#### Failing Cases (Synced Data)
- **Vibe IDs**: `fmehj91sfrrqff8cg8`, `fmehj9hy1pwdy0ddrx`
- **Error**: `Failed to get Screenshot size: Error: {"module":"EncryptedBlockstore"...}`
- **Display**: File attachments don't render (fail silently)
- **Database**: Cloud-synced data (`userId = 'zs4m1mTYGk5LvSRsw'`, with toCloud attachment)

### Code Context

The error occurs when calling `file()` method on Fireproof file attachments:

```typescript
// In React component - accessing synced file attachment
if (doc._files?.screenshot) {
doc._files.screenshot.file().then((file: File) => {
console.log(`Screenshot size: ${file.size}`);
}).catch((error: Error) => {
console.error(`Failed to get Screenshot size:`, error);
});
}
```

### Database Configuration

```typescript
// Conditional toCloud() attachment based on userId
const { database, useAllDocs } = useFireproof(dbName,
userId && userId !== 'local' ? { attach: toCloud() } : {}
);
```

### Console Error Details

```
[ERROR] {module: EncryptedBlockstore, this: z2z4uNe, level: error, cid: baembeigd3itv3ao4ub2xrid25e5...}
[ERROR] {module: EncryptedBlockstore, this: z2z4uNe, level: error, cid: baembeic6lyzpazbufr6wutswdys...}
[ERROR] Failed to get Screenshot size for fmehj91sfrrqff8cg8: Error: {"module":"EncryptedBlockstore"...}
[ERROR] Failed to get Screenshot size for fmehj9hy1pwdy0ddrx: Error: {"module":"EncryptedBlockstore"...}
```

Multiple failing CIDs with prefixes: `baembeigd3itv3ao4ub2xrid25e5`, `baembeic6lyzpazbufr6wutswdys`, various `bafkr` CIDs.

### Expected Behavior
File attachments should be accessible via `file()` method regardless of whether data is local or synced via `toCloud()`.

### Actual Behavior
- ✅ Local file attachments work correctly
- ❌ Synced file attachments throw `EncryptedBlockstore` errors
- ❌ "car not ready" messages suggest CAR (Content Addressable aRchive) format issues
- ❌ File attachment data becomes inaccessible after sync

### Reproduction Steps
1. Create document with file attachment locally (works fine)
2. Enable cloud sync using `toCloud()` attachment
3. Access file attachment via `doc._files.attachmentName.file()` (fails)
4. Observe EncryptedBlockstore errors in console

### Impact
- File attachments become inaccessible after cloud sync
- Breaks user experience for cloud-synced applications
- Suggests broader file attachment synchronization issues

### Questions
1. Is there a known issue with file attachment sync in v0.23.0 upgrade branch?
2. Are CAR format files properly handled during `toCloud()` sync?
3. Should `EncryptedBlockstore` errors be expected for synced content?
4. Is there a different API for accessing synced file attachments?
5. Are there any workarounds for accessing synced file data?

### Additional Context
This issue affects the Vibes DIY application where screenshots are stored as Fireproof file attachments. The problem only manifests when using cloud sync - local-only databases work perfectly.