To increase the upload size limit, edit the PHP configuration file.
If you using php7.4 then replace 8.1 to 7.4 in the below commands
sudo nano /etc/php/8.1/fpm/php.ini
Find the following line.
upload_max_filesize = 2M
Change the value like below. Note that this value should not be larger than the attachment size limit set by Postfix SMTP server.
upload_max_filesize = 50M
Then find the following line.
post_max_size = 8M
Change the maximum size of POST data that PHP will accept.
post_max_size = 50M
Save and close the file. Alternatively, you can run the following two commands to change the value without manually opening the file.
sudo sed -i 's/upload_max_filesize = 2M/upload_max_filesize = 50M/g' /etc/php/8.1/fpm/php.ini sudo sed -i 's/post_max_size = 8M/post_max_size = 50M/g' /etc/php/8.1/fpm/php.ini
Then restart PHP-FPM.
sudo systemctl restart php8.1-fpm
Nginx also sets a limit of upload file size. The default maximum upload file size limit set by Nginx is 1MB. If you use Nginx, edit the Nginx configuration file.
sudo nano /etc/nginx/conf.d/mail.example.com.conf
Add the following line in the SSL virtual host.
Save and close the file. Then reload Nginx for the changes to take effect.
sudo systemctl reload nginx