You need a new mimetype for bluej files. Start a simple test to see why:
$ mimetype pacjage.bluej
pacjage.bluej: text/plain
Therefore create a new mime type
Create a new configuration via
nano ~/.local/share/mime/packages/bluej.xml
Add the configuration below
<?xml version="1.0" encoding="UTF-8"?>
<mime-info xmlns='http://www.freedesktop.org/standards/shared-mime-info'>
<mime-type type="text/bluej">
<comment>bluej file</comment>
<glob pattern="*.bluej"/>
<glob pattern="*.BLUEJ"/>
</mime-type>
</mime-info>
Update the mime database
update-mime-database ~/.local/share/mime
Check the mimetype of your bluej
file once again
$ mimetype pacjage.bluej
pacjage.bluej: text/bluej
For a system-wide configuration use the configuration file
/usr/share/mime/packages/bluej.xml
and update with
sudo update-mime-database /usr/share/mime