I recently switched to GNOME 3 and was surprised to find that there was no way by which we could set the default applications. I am writing a Nautilus extension using Bash. I want to find the MIME type of a file. 

Initially I used to extract the extension of the file using `sed` and build the code on it. Then I realized that there is this command called `file`. When I try to find the `mime-type` of a `mkv` file, the command `file --mime-type -b <filename>` outputs `application/octet-stream` but when I see the Nautilus properties window it shows the correct `video/x-matroska` mime-type. 

Am I missing anything here? If not is there a better way in which I can file the mime-type of a file?