Can't edit the "application/x-linguist" MIME type

I am trying to get Qt Creator to highlight *.ts files as JavaScript files. I opened the options and went to MIME Types. I first added *.ts to the "application/javascript" MIME type. That didn't help. Then I went through all MIME types to see if any of them already include *.ts. I noticed that the "application/x-linguist" MIME type has it. I tried editing the patterns of it to remove *.ts but every time I click apply, the selection of the MIME type is cleared, then when I select "application/x-linguist", I notice that my changes were not saved.