All programs become untrusted after they reach a certain size. It's highly unlikely that a 1,000,000 line monstrosity with a file format so complex no single hu

All programs become untrusted after they reach a certain size. It's highly unlikely that a 1,000,000 line monstrosity with a file format so complex no single human can understand has no exploitable flaws in its file handling. With "open by extension" the .pdf file can only target vulnerabilities in my PDF reader, but with "open by mimetype" they could be trying to exploit any program that is configured to open files. If you're doing "open by extension" and know that Adobe's exquisite PDF reader will open when you double click a PDF, doing so is not more unsafe than opening up Reader and navigating to the file from within. That's without mentioning the main obvious advantage of "open by extension" - the fact that I can configure programs that open file formats that haven't been blessed by whoever wrote your `file` utility.