Middleware to detect if a POST parameter of a given type is present, and if it is a data URI string we convert it to an instance of UploadedFile and make it accessible via Laravel's Request class and the file handling methods there.

DataUriToFile.php

<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Http\UploadedFile;

class DataUriToFile
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @param  string  $field
     * @return mixed
     * @example Route::post()->middleware('DataUriToFile:image')
     */
    public function handle($request, Closure $next, string $field)
    {
        if ($request->has($field) && substr($request->{$field}, 0, 11) === 'data:image/') {
            $dataUri = $request->{$field};
            $tempFile = tempnam(sys_get_temp_dir(), 'laravel-data-uri-');

            $file = file_get_contents($dataUri);
            file_put_contents($tempFile, $file);
            $fileMime = mime_content_type($tempFile);

            // Add the converted file to request object
            $request->files->add([
                $field => new UploadedFile($tempFile, 'image', $fileMime, strlen($file)),
            ]);

            // Unset/remove the original parameter
            $request->offsetUnset($field);
        }

        return $next($request);
    }
}

Please note that this does not currently work.

It does now!