The "/ext" page:
The /ext page is only functional if the "extroot: DIR" setting is found in the CGI script that launched Fossil, or if the "--extroot DIR" flag is present when Fossil is lauched using the "server", "ui", or "http" commands. DIR must be an absolute pathname (relative to the chroot jail) of the root of the file hierarchy that implements the CGI functionality. Executable files are CGI. Non-executable files are static content.
The path after the /ext is the path to the CGI script or static file relative to DIR. For security, this path may not contain characters other than ASCII letters or digits, ".", "-", "/", and "_". If the "." or "-" characters are present in the path then they may not follow a "/".
If the path after /ext ends with "/" and is the name of a directory then that directory is searched for files named "index.html", "index.wiki", and "index.md" (in that order) and if found, those filenames are appended to the path.