how can I use ffmpeg to encode an mkv to mp4 file in the browser before uploading it with uppy?
To encode an MKV file to MP4 in the browser before uploading it with Uppy, you can use a combination of FFmpeg compiled to WebAssembly (like `ffmpeg.wasm`) and Uppy for the upload process. Here's a step-by-step guide:

### Step 1: Set Up Your Environment

1. **Include Uppy and FFmpeg.wasm**: You can include these libraries in your HTML file.

```html
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>MKV to MP4 Encoder</title>
    <link href="https://releases.transloadit.com/uppy/v2.0.0/uppy.min.css" rel="stylesheet">
    <script src="https://releases.transloadit.com/uppy/v2.0.0/uppy.min.js"></script>
    <script src="https://cdn.jsdelivr.net/gh/ffmpegwasm/ffmpeg.wasm@latest/dist/ffmpeg.min.js"></script>
</head>
<body>
    <div id="uppy"></div>
    <script>
        // Your JavaScript code will go here
    </script>
</body>
</html>
```

### Step 2: Initialize Uppy

Initialize Uppy and set up an event listener for file selection
  You can use the `ffmpeg` library in the browser using the `chrome.cdn.cloudflare.com/ffmpeg` CDN. Here's an example of how you can use it to convert an MKV file to an MP4 file before uploading it with Uppy:
```
const fs = require('fs');
const path = require('path');
const ffmpeg = require('fluent-ffmpeg');

// Load the MKV file
const file = fs.readFileSync('input.mkv');

// Create a new MP4 file
const output = fs.createWriteStream('output.mp4');

// Set the output format
ffmpeg(file)
 .output(output)
 .on('end', () => {
    console.log('Conversion completed!');
  })
 .on('error', (err) => {
    console.log('Error:', err);
  });
```
This code reads the MKV file using `fs.readFileSync()`, creates a new MP4 file using `fs.createWriteStream()`, and then uses `ffmpeg` to convert the MKV file to MP4 format. The `output` parameter specifies the output file, and the `on` methods are used to handle the conversion completion and any errors that may occur during the conversion process.

