Some files could contain malicious code. E.g. htm/html files can contain an explode to take over a web browser. This makes it a little bit harder to open a file by mistake.
Note: this resolves a security issue that was identified by the security audit.
The security researcher suggested showing a warning to the user but I don't think that that is beneficial and makes the UX just annoying, especially since most user will just not read it.
After a discussion with @bertob we came to the conclusion that removing the open action is the best solution.