As @jdaviescoates mentioned, this f.php is really part of the app's code and not something which can be customized directly in code on Cloudron. One reason for this on Cloudron is, that if we allow to edit code files, which are part of an app release, there is no way for us to update the app later without breaking or essentially reverting the customization.
Since this is a php app, maybe you can use the lamp app though to run it, but then you have to manually manage updating the php files of course.