The operating principle is quite simple. After installation, you can simply add the Multi Html File Selector attribute type. When adding, you can specify the maximum number of files to be uploaded. You can do this in the attribute settings.
On the "Multi Html File Selector> Settings" dashboard page, you can also define an upload limit per IP address. This will protect you from spamming your file system. The maximum upload limit, however, can be changed in your PHP settings if necessary.
You can regulate permitted file types on the concrete5 "Allowed file types" dashboard page.
In addition, a Cleaner Job is included in this extension. This is necessary in order to comply with the GDPR inasmuch you have included the upload attribute into public express forms. You should automatize this task via Cronjob periodically (e.g. every hour). This Job removes all unprocessed data (residual data) from cancelled forms.