Help with webshell PHP file

I have a PHP webshell. The shell is without uploading a file from local and as far as download it can only view it or keeps it in the browser. The solution would need to work over URL without any user interface. The download is infact working if it is a text that is safe without MIME type because it has no autodetect of MIME or MIME header setting at runtime which I think will solve the problem. The goal is to keep a local folder and file repository in sync with the host, while being able to facilitate an online presence between the two. The site will be a CMS for invoices on a basic realm subdomain. We know that clientside software can do everything we need but we are open to solutions to keeping in an online presence, to better centralize access and availability. Upload should be in javascript and it's possible that once the file has been saved to host that it should be modified with the local file's date modified. Download works and it may be just that it needs to be built upon for a javascript front end.