I would suggest to order audit on everything this one do and did in the company.
Dishonesty tends to manifest in more than one way.
You may find several reasons of firing him for cause and start legal proceedings.
Even If not - fire him in a way that limits his opportunity to harm your company and its products.
Complete offsite backup, disabling all his ...
sf02 has a good answer, but I would modify the actual request:
Hello X. I need file (include filename) as soon as possible so I can (do something with it, hopefully something that X thinks is important too). If it's not processed by (date), then (these are the consequences). If you still need something from me first, please let me know, and I will take ...
You have already sent a reminder with no response. I would reply to your last reminder and CC your boss. You can politely ask again for the status of your request with something like:
Hello X, just following up on this request. Let me know if you need any additional information. Thanks.
If after this you still do not hear back from this colleague then ...