To configure the "Customer Files Uploads & Product Attachments" add-on, open the settings page from your store's App Store:
You will see a page like this:
Max number of files that can be attached: Specify the maximum number of files that a customer can add.
Allowed file extensions: Provide a comma-separated list of file extensions that should be allowed for upload, for example, JPG, TXT, SVG.
Max file size (MB). Set to "0" (zero) for unlimited: Set the maximum file size.
Note that the PHP settings upload_max_filesize and post_max_size may impose their limitations on the maximum size of file uploads if they are lower than those specified in the add-on settings. To increase these parameters, edit the file php.ini or contact your server administrator for assistance.
Click "Submit" to make the settings active.
Now you can proceed to enable customer file attachments for products.