goobi-viewer-core: Path traversal via HTTP `Content-Disposition` header in ExternalFilesDownloader enables remote code execution in Goobi viewer

The Goobi viewer is a web application that allows digitised material to be displayed in a web browser. From version 24.03 to before version 26.05, ExternalFilesDownloader.getFilename() extracts the destination filename from a server-supplied Content-Disposition response header and passes it directly to Path.resolve() without stripping directory components. This allows for a malicious HTTP server that the viewer fetches from to supply an absolute path or ../ traversal sequences as the filename, writing attacker-controlled content to an arbitrary path writable by the Tomcat process. When targeting the deployed web-application directory, this yields remote code execution as the tomcat OS user. This issue has been patched in version 26.05.