I am getting the post too large exception when I try to upload files which are more than 2 MB in size and I've already tried everything like increasing the post_max_size and max_upload_size in php.ini file but I am still getting the same error. When I comment the handler function in validatepostsize.php file it shows token mismatch error:

I think you can check the following parameters in your php.ini file like:
upload_max_filesize = 40m
post_max_size = 50m
After restart your server like (apache)
Hope this work for you!
First, check your PHP version.
php -v
The command below will print the path to the php.ini file that your server is using.
php -i | grep php.ini
Next.
sudo nano /etc/php/7.4/cli/php.ini
The values of post_max_size, upload_max_filesize and memory_limit by default have the value of 8M, 2M, and 128M respectively.
Search for those variables and change their values, whilst ensuring that the sizes follow the same ratio as the default values.
See example below:
post_max_size = 2G
upload_max_filesize = 1G
memory_limit = 3G
For "heavy-loaded sites", it is useful to have FPM installed and running.
sudo apt install php7.4-fpm -y
sudo service php7.4-fpm start
Finally, restart your web server.
sudo service apache2 restart
One of the issues is that the Laravel PHP artisan serve local development server uses the php.ini under PHP/CLI instead of under PHP/apache2
So if you are having this issue while developing with PHP artisan serve, go and fetch the php.ini that's under the /etc/php[version]/cli folder.