P-touch Editor Label
This format is used by Brother's P-touch Editor range of label-making software. .lbt files appear to be created by P-touch Editor Lite, with .lbx files created by the fully-featured editor.
The format appears to store a range of information about the contents of the label (text / image, formatting information), as well as information about the labelling device used and the label itself (length / width). A sample .lbt file can be found here