File Upload Testing Guide & Checklist

File upload is one of the most complex features to implement securely. It involves frontend handling, network transmission, backend storage, and often file processing. Here is the definitive checklist to ensure your implementation is bulletproof.

1. Functional Testing (The Basics)

Ensure the core functionality works as expected.

✅ Single File Upload: Can I upload a standard JPG?
✅ Multiple File Upload: Can I upload 5 files at once? (If supported)
✅ Cancel Upload: Can I cancel an upload in progress?
✅ File Preview: Does the thumbnail appear correctly before upload?
✅ Drag & Drop: Does dragging a file works as well as clicking 'Browse'?

2. Boundary Value Analysis (Size Limits)

Test the edges of your size restrictions. Use our Generator Tool for precision.

✅ Maximum Size Limit: If limit is 10MB, try uploading 10.1MB. It should fail immediately.
✅ Zero Byte File: Upload a file with 0 bytes. Most systems choke on this.
✅ Large Files: Test a file slightly smaller than the limit (e.g., 9.9MB).

3. File Type & Extension Testing

This is where most security bugs live.

✅ Allowed Extensions: If only `.pdf` is allowed, try uploading `.txt`.
✅ Case Sensitivity: Try `image.PNG` vs `image.png`.
✅ Double Extensions: Try `malware.php.jpg`. Does the server execute it?
✅ No Extension: Upload a file named just `file`.
✅ Long Filenames: Upload a file with a 255+ character name. Does the database truncate it?

4. Security & Malware

Never assume an uploaded file is safe.

✅ Malicious Scripts: Try uploading a harmless text file containing pseudo-code script tags.
✅ SQL Injection in Filename: Name a file `Robert'); DROP TABLE Students;--.jpg`.
✅ Mime-Type Spoofing: Change a `.exe` file to `.jpg` and upload. Ideally, the backend should detect the header signature mismatch.

5. Network & Performance

✅ Network Interruption: Disconnect Wi-Fi mid-upload. Does the app crash or show "Connection Lost"?
✅ Slow Connection: Throttle network to 3G. Does the timeout trigger appropriately?

Summary

A robust file upload system handles all these edge cases gracefully. It provides clear error messages to the user ("File too large", "Invalid format") rather than generic "Server Error" codes.

Use the Dummy File Generator to create the test assets needed for steps 2 and 3 quickly.