When a new image file is detected via Image::pathCreated(), its file extension is checked against BackgroundFinder::suffixes() to ensure that it is a supported format. However, despite its name, suffixes() returns a list of glob patterns (i.e. all prefixed with "*.") so the contains() never succeeds and the new file is not added.
suffixes() is also used within the BackgroundCreator class itself to set a directory name filter. To maintain compatibility it may be better to not change the name or meaning of this function, but instead to take account of the pattern in Image::pathCreated(). This patch does just that.