Most online file converters work the same way: you pick a file, it uploads to their server, their server runs conversion software, and you download the result. This model is simple to build, and for many years it was the only practical option. Then WebAssembly arrived — and with it, the ability to run genuinely powerful software directly inside a web browser. Today, the upload-to-server model is not just outdated; for most common conversion tasks, it's objectively worse for users.
What Actually Happens When You Upload a File
Sending a file to a conversion server means trusting them with more than you might realise:
- Your file travels over the internet, encrypted in transit but readable at the destination
- It's stored on their server — sometimes for hours to enable download links, sometimes indefinitely
- Their privacy policy may allow using your file data for analytics, advertising targeting, or AI training
- Their security practices become your problem — a breach of their server exposes your files
- If the service is free, you're paying with your data in ways that may not be clearly disclosed
- You join a processing queue — their server load directly affects how long you wait
- The service can see the contents of what you convert — your tax return, medical document, private video
How Browser-Based Conversion Works
Convifi uses WebAssembly (WASM) — a format for running compiled native software inside the browser at near-native speed. When you convert a file, here's what actually happens:
- You select a file — it remains on your device, in browser memory
- Your browser downloads the conversion engine (FFmpeg, Pandoc, libvips, or a background removal AI) — once, then cached in your browser cache
- The conversion engine executes entirely inside your browser's sandboxed environment
- Your file data is processed in memory — zero network requests are made with the file content
- The result file is generated locally and a download link is created pointing to your browser's memory
- The download happens — no server involved at any point
The Privacy Case
What that means in practice, by file type:
- Video recordings: meetings, personal videos, sensitive professional content stay off any server
- Document conversion: contracts, legal documents, financial reports, medical records are never transmitted
- Image processing: personal photos, ID documents, passport photos, sensitive business images are processed locally
- Audio extraction: private recordings, voice memos, proprietary content never leaves the device
- Background removal: face photos and personal images are processed by a model on your CPU/GPU, not anyone's cloud
Is It Actually Faster?
For anything above a few megabytes, local is usually faster door-to-door. Here's the maths:
- Upload latency: a 500 MB MKV on a 50 Mbps connection takes 80 seconds of uploading before conversion begins
- Queue time: popular free converters can have queues of 5–30 minutes during peak hours
- Download time: another 80 seconds to download the result
- Total for server-based: 80s + queue + 80s = easily 10+ minutes for a large file
- Local processing: starts immediately, with your CPU doing work. A 500 MB MKV to MP4 on a modern laptop takes 2–5 minutes total.
- The tradeoff: your CPU vs their server CPU. For small files (under 50 MB), the upload/download overhead is minimal and server-based is comparable. For large files, local wins clearly.
The WebAssembly Ecosystem Today
WebAssembly has been production-ready since about 2021. The range of software running in the browser today is genuinely surprising:
- FFmpeg: An open-source video/audio conversion tool that supports virtually every codec and container format you're likely to run into.
- Pandoc: Universal document conversion between 40+ formats. Used in Convifi for Markdown, HTML, DOCX, EPUB, LaTeX, and more.
- libvips: A professional image processing library — faster than ImageMagick for most tasks, 5x smaller WebAssembly bundle.
- TensorFlow.js and ONNX Runtime: Machine learning inference for tasks like background removal, image upscaling, and object detection.
- SQLite: Full relational database running in the browser — used by apps like Notion for offline mode.
When Server-Based Conversion Still Makes Sense
That said, there are real limits:
- Very long videos (1+ hour): Modern browsers have memory limits that can be exhausted by very large files. A 10 GB movie file may fail on some systems.
- Batch processing: Converting 500 files one-by-one in a browser is impractical. Server-side batch APIs are better for bulk work.
- Exotic formats: Some obscure codecs and container formats require codec libraries too large to ship as WebAssembly bundles.
- Low-powered devices: Very old phones and budget tablets may lack the RAM or CPU to run WebAssembly conversion within reasonable time limits.
- For everyday conversions — video clips under 2 GB, images, audio files, and documents — browser-based conversion is the superior choice in almost every way.
How to Verify That No Upload Happens
If you're sceptical — which is healthy — you can verify Convifi's no-upload claim yourself. Open your browser's DevTools (F12 in Chrome), go to the Network tab, and start a conversion. Watch the network requests: you'll see the WASM engine files load, and then nothing — no request containing your file data, no POST to a conversion API, no upload progress indicator in the network waterfall. Your file never leaves the browser's memory sandbox.