Most special pages which list images (MostLinkedFiles, MostGloballyLinkedFiles, NewFiles, UncategorizedFiles, UnusedFiles) generate a caption from the filename and some basic information such as image size. MediaViewer picks this up and displays it as a caption; it should fall back to showing the file page description instead (which it does when there is no caption at all).

This is a more generic version of T77691.