multipart/form-data broken on new firefox version
Multipart forms seem to be broken in new firefox versions, no idea why. It works in chromium, qtwebkit and firefox 128 ESR but it seems like something changed in a new version. I looked at the difference between the form format for between firefox versions but they seem the same. Here's a minimal example:
```nim
import prologue

const page = """<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
    <form action="/upload" method="post" enctype="multipart/form-data">
        <input type="file" name="image">
        <button type="submit">Upload</button>
    </form>
</body>
</html>"""

proc form(ctx: Context) {.async.} =
    resp htmlResponse(page)
    
proc upload(ctx: Context) {.async.} =
    try:
        let f = ctx.request.formParams["image"]
        if f.body.len == 0:
            resp "<h1>File upload is empty.</h1>"
        else:
            resp "<h1>Success, filename: " & f.params["filename"] & "</h1>"
    except KeyError:
        resp "<h1>Error, param 'image' not found.</h1>"

var app = newApp()
app.addRoute("/", form)
app.addRoute("/upload", upload, HttpPost)
app.run()
```
Will always respond with the error on firefox 135