The charm allows for some settings for php. The most usual scenario is to allow for larger file uploads.
This is completely site dependent, but this is an example for allowing 2G file uploads.
juju config nextcloud php_upload_max_filesize=2G juju config nextcloud php_post_max_size=2G juju config nextcloud php_memory_limit=2G
There are a few more config settings in the charm that affects how nextcloud manages up/downloads which you might want to check.