In our BSP documentation, buildfiles, and scripts, we use a filename convention that relies on prefixes and suffixes to distinguish between file types. The names for Image Filesystem (IFS) files take the form ifs-board_name.elf, and the names for Embedded Filesystem (EFS) files take the form efs-board_name.raw.
The IDE uses a somewhat simplified filename convention. Only a file's three-letter extension, not its prefix or any other part of the name, determines how the IDE handles the file. For example, an OS image file is always an .ifs file in the IDE, regardless of its format: ELF, binary, SREC, etc. To determine a file's format, you need to view the file in an editor.