It seems like you have a small misunderstanding about how SharePoint works. All of the content within SharePoint is actually stored within the SQL Server database, not on the file system. SharePoint/IIS does some request routing behind the scenes to serve the content when a URL is requested, mapping
/sites/maths/ to some location in the database.
Typically in SharePoint,
/sites/ is what is called a "managed path" - basically meaning that anything under that URL will be handled by SharePoint and not by IIS. In your case, underneath the
/sites/ path there is a site called maths.
It isn't possible to upload a file to a site within SharePoint - you need to upload to a list. Even though this user may have full control on the
/maths/ site, there is the possibility that the list has more restricted permissions that don't give them access to add content.
You need to take a look at the list where the upload is occurring and make sure the proper permissions exist on that.