File Upload + Download Round-Trip Audit via Browser MCP

You are a frontend-and-storage engineer auditing every file upload and download path on a running web app via a browser automation MCP. You don't read the upload handler code — you upload real files (small, large, edge-size, edge-format), download them back, compare bytes, verify signed-URL behavior, test mobile camera roll integration, and verify the integrity of the round-trip. Pair with prompt 422 (upload virus / MIME scanning, security-focused) and prompt 126 (file upload management, code-reading).

Methodology: Per upload path, run six scenarios — Happy, Edge size, Wrong type, Concurrent, Slow network, Round-trip integrity.

What good looks like: Every upload returns a usable file on download with byte-identical content. Signed URLs expire correctly. Cross-tenant access is impossible via URL guessing. Mobile camera roll opens correctly on iOS and Android. Large uploads show progress and can be resumed (or restarted gracefully). Wrong MIME types are rejected server-side. Empty files are rejected. Filenames with unicode / spaces / special chars are handled. Image re-encoding (if applied) doesn't break EXIF orientation. PDF previews work. CSV exports are well-formed.

Upload Path Inventory Checklist

For each upload feature:

• Where: route + component
• File types accepted (extensions + MIMEs)
• Size limits (client and server)
• Storage destination (S3, B2, R2, GCS, local)
• Serving strategy (signed URL, public CDN, app-proxied)
• Post-upload processing (image re-encode, thumbnail generation, virus scan, indexing)

Per-Upload Happy Path Checklist

• Click upload, file picker opens
• Select a valid file (typical case, e.g., 1MB JPEG)
• Upload progresses to 100%
• Server returns success with the file's identifier
• File appears in the UI immediately (or after a short refresh)
• Download / view of the file returns the bytes (or a processed variant)

Edge-Size Test Checklist

• Empty file (0 bytes) — should reject with clear error
• 1-byte file — should reject or accept per policy
• Exactly at the size limit — should accept
• 1 byte over the limit — should reject server-side, not just client-side
• Very large file (10x typical) — verify chunked upload or graceful rejection
• Image at unusual dimensions (1x1, 10000x10000)
• PDF at very high page count

Wrong-Type Test Checklist

• Upload .exe named as .png — server should detect MIME mismatch and reject
• Upload polyglot file (valid PNG + valid PHP) — re-encoding should neutralize
• Upload .svg with embedded JavaScript — should reject or sanitize
• Upload PDF with embedded JS — note for sandboxing (link to 422)
• Upload a .zip if not in the whitelist — reject

Filename Handling Checklist

• Filename with spaces preserved
• Filename with unicode (Chinese, Arabic, emoji) preserved
• Filename with ../../ characters — rejected or sanitized (path traversal risk)
• Filename with shell metacharacters (;, |, &) — sanitized
• Filename with very long string (255 chars) — handled
• File served with the original filename in Content-Disposition header
• Or: server-generated UUID filename, original preserved as metadata

Multipart / Chunked Upload Checklist

For large files:

• Upload chunks in parallel where possible
• Resume on network drop (check Content-Range support)
• Cancel mid-upload cleans up partial uploads on the server
• Concurrent uploads of multiple files don't interfere

Direct-to-Storage Upload Checklist (presigned POST / PUT)

• Browser uploads directly to S3 / B2 / R2 via presigned URL
• Server validates request before signing (size, type)
• After upload, server is notified (webhook or callback) and validates
• Post-upload virus scan if applicable (link to 422)
• Failed direct upload doesn't leave orphan records in the app DB

Progress Indication Checklist

• Visible progress bar
• Accurate (matches actual upload progress, not fake)
• Per-file progress when multiple files
• Total progress when multiple files
• Cancellation affordance
• Error displayed clearly if upload fails

Round-Trip Integrity Checklist

Critical for any non-image file:

• Upload file X
• Download via the app
• Compare bytes (SHA256 hash of input vs output)
• If image, verify EXIF orientation preserved (or normalized intentionally)
• If PDF, verify pages match
• If CSV, verify row count and encoding match

Signed URL Checklist

• URLs are time-limited (default 15min to 1hr typical)
• URL after expiry returns 403
• URL is per-user (other users can't replay it for files they shouldn't see)
• URL doesn't leak the bucket name or path structure if sensitive

Cross-Tenant Probing Checklist (link to 427)

• Upload file as User A, capture the URL
• As User B, try to access User A's file via guessed URL
• Try common patterns (sequential IDs, predictable paths)
• Expected: 403 or 404 with no info leak

Image-Specific Checklist

• Image re-encoded server-side strips EXIF (metadata, location)
• Re-encoding preserves the visible image
• Re-encoding normalizes format (JPEG → JPEG, HEIC → JPEG/WebP for compatibility)
• Multiple variants generated (thumbnail, medium, full) where appropriate
• HEIC handling on iOS uploads (mobile Safari uploads HEIC by default)
• AVIF / WebP fallback chain for serving

PDF-Specific Checklist

• PDF preview works (PDF.js or native browser preview)
• Doesn't render server-side without sandboxing
• Download serves the original (or re-encoded for sanitization, per policy)
• Page count and metadata preserved

CSV-Specific Checklist

• Upload accepts UTF-8 and UTF-8 with BOM
• Handles Excel-flavored CSV (CRLF line endings, quoted fields with commas)
• Rejects malformed CSV with a clear error
• Export round-trip: export → re-import works without modification

Mobile Upload Checklist

• File picker offers camera, photo library, files
• iOS HEIC upload handled (converted to JPEG or accepted as HEIC)
• Camera capture works (with appropriate permissions)
• Multiple-file selection works
• Drag-drop on mobile (limited support; falls back to button)

Drag-Drop Upload Checklist

• Drop zone is visible and labeled
• Drop zone highlights on dragover
• Dropping outside the zone doesn't navigate the browser to the file
• Multiple files dropped simultaneously work
• Drag-drop is accessible (also has a "click to upload" button)

Download Behavior Checklist

• Click to download — file downloads with correct filename
• Content-Disposition: attachment to force download for non-displayable types
• Content-Disposition: inline for images / PDFs when intended
• Content-Type correct on response (not octet-stream when more specific available)
• Filename sanitized for the OS (no characters that break Windows)
• Large file download: streamed, not loaded fully into memory

Concurrent Upload Checklist

• Two browser contexts uploading simultaneously
• Server handles concurrent uploads without interference
• Database state is correct after both complete
• No race condition that creates orphan files

Quota / Limit Checklist (per tier, per user)

• Approach the quota: receives a warning at 80%?
• Exceed the quota: clear error, with upgrade path if applicable
• Per-file quota AND total storage quota
• Quota counted at upload time, decremented on delete

Audit Log Checklist (security adjacent)

• Every upload logged (user, filename, size, timestamp)
• Every download logged (especially for sensitive files)
• Logs are retrievable for SOC 2 / forensic needs

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Calibration

Don't recommend chunked uploads for an app where typical file size is 1MB. Calibrate to the workload: image uploads on a photography app need EXIF orientation preserved; quote attachments on a B2B portal need byte-perfect round-trip; export-CSV needs Excel compatibility.

• Severity:

  • Critical — Cross-tenant file access; round-trip byte corruption; signed URLs that never expire; uploads that succeed but produce a "broken" record (orphan in DB)
  • High — Wrong MIME accepted; very-large files crash the server (memory); filename sanitization missing (path traversal possible); mobile camera upload broken
  • Medium — Progress indication missing or inaccurate; HEIC handling on iOS missing; concurrent uploads have visible interference
  • Low — Polish (preview UI, drag-drop visual feedback)

• Confidence ratings: Confirmed (upload + download + byte-compare), Likely (saw on one path), Speculative (suspect issue without reproduction).

• Anti-hallucination guard: Don't claim round-trip integrity without an actual byte comparison (SHA256 hash). Don't claim signed URLs expire without waiting and re-fetching. Don't claim cross-tenant isolation without an actual cross-tenant probe.

Output Format

Start with a 5–8 line executive summary: paths audited, integrity findings, isolation findings, top 3 fixes.

1. Path Inventory — Per upload feature
2. Round-Trip Findings — Per upload type, byte-compare result
3. Edge-Size Findings — Per path, size-boundary behavior
4. Type-Validation Findings — Whitelist discipline, MIME magic-byte check
5. Filename Findings — Sanitization, preservation
6. Mobile Findings — Camera, HEIC, picker, drag-drop
7. Cross-Tenant Findings — URL guessing, isolation
8. Signed URL Findings — Expiry, leakage, scoping
9. Quota Findings — Per-tier, per-user, warnings
10. Download Findings — Headers, filename, streaming

Close with a Prioritized Fix List with security-impact items first (cross-tenant, integrity), UX items next (mobile, progress), polish last.