In general, I think of this as a feature that reduces the amount of horizontal space used by filename tabs. In the case of "header.png", the lone ".h" indicates "bootm.h". In the case of "source.png", I'm not sure what the filenames actually were, but it looks like two different files with the name bootm.c adjacent in the tab list.
There is an option to enable or disable this behavior: Tools, Options, Editing, Editor Windows, Abbreviate File Tab Captions.
