Rework the modal message for Updating Filetypes

[dmols]

With the changes made in #900, the user will now be allowed to update files with more modern ones. Doing so will keep the file system in the course up to date, but it will also prevent users from uploading older filetypes to replace modern ones. We want to mention that the user should upload a more modern file, or update their choice of program so their latest changes on there are saved with a modern filetype.

This documentation change would ensure the user knows the proper steps to replace the outdated file, with a newer one.