Ubuntu – How to get the mime type of a file from the command line

You need to:

1. Add a new MIME type

2. Associate that MIME type with a program (see man xdg-mime for more details):

   xdg-mime default APPLICATION MIMETYPE(S)
   

You can add mime types to a system by using xdg-mime.

command line tool for querying information about file type handling and adding 
descriptions for new file types

Command to add a new mime type:
xdg-mime install [--mode mode] [--novendor] mimetypes-file

From the man page:

mode: can be user or system; for system you need root permissions.

install: Adds the file type descriptions provided in mimetypes-file to the
desktop environment.  mimetypes-file must be a XML file that
follows the freedesktop.org Shared MIME-info Database specification
and that has a mime-info element as its document root. For each new
file type one or more icons with name type-subtype must be
installed with the xdg-icon-resource command in the mimetypes
context. For example the filetype
application/vnd.oasis.opendocument.text requires an icon named
application-vnd.oasis.opendocument.text to be installed (unless the
file type recommends another icon name).

Alternatively you could use the ubuntu wiki on mime types. This uses manual instructions to edit in this text

gksudo gedit /etc/mime.types

and to edit it something like

text/extension extension

This editing of /etc/mime.types can also be accomplished with the command line stream line editor sed but I would suggest using xdg-mime.