alf.io vulnerable to Arbitrary File Read and Exfil via simpleHttpClient Extension Script

alf.io is an open source ticket reservation system for conferences, trade shows, workshops, and meetups. Prior to version 2.0-M5-2606, the alf.io extension sandbox injects a fully-functional HTTP client (`simpleHttpClient`) into every extension script's scope. The `postFileAndSaveResponse()` method accepts an arbitrary filesystem path as its `file` parameter and reads the file contents using `new FileInputStream(file)` with no path validation, directory restriction, or allowlist. A malicious extension script can read any file accessible to the JVM process user and exfiltrate it to an attacker-controlled server via HTTP POST. Version 2.0-M5-2606 patches the issue.