You’re probably familiar with the Identify option on the Tracker’s context menu — it tells the Registrar to try and assign a MIME type to a file if it doesn’t have one already.
You can run the exact same process from the Terminal, with one of these two commands:

mimeset -f filename
mimeset -F filename

The first one functions exactly like Identify, while the second will change the file’s type if it’s not correct. The nice thing about these as opposed to using Identify is that you can run them from within scripts, or even over your entire hard drive.