If you take only the leading part of jpg or jp2 or webp file, you'll get image with missing parts or decoding will fails entirely.
When I experimented with LuraWave special proprietary lwf, file truncation only decreased quality uniformly (as if the picture were encoded with lower quality settings initially)
What other picture formats supports this, i.e. allows graceful degradation when tail of file is missing?
The main idea is to store images with high quality and later be able to quickly reduce quality (and file size) bit by bit without re-encoding.