returns a list of all pdf, gif and jpg-files from fileadmin/files/ sorted by their name reversely and with the full path (with "fileadmin/files/" prepended).
fileadmin/files/ | pdf,gif,jpg | name | r | true
[path relative to the web root of the site] | [list of valid extensions] | [sorting: name, size, ext, date] | [reverse: "r"] | [return full path: boolean]
Files matching are returned in a comma-separated string.
The value of config-option "lockFilePath" must equal the first part of the path. Thereby the path is locked to that folder.