/usr/share/xbubble/fonts.dir is in xbubble-data 0.5.11.2-3.3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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1 baskvli.pfb -freefont-Baskerville-normal-i-normal--0-0-0-0-p-0-iso8859-1