wrong_extension

Five files whose name lies about what is inside: a JPG named .mp3, an MP3 named .jpg, an MP4 named .jpg, an MP4 named .png, and a PNG named .mp4. Checking only the extension is one of the commonest upload mistakes, and it lets people sneak the wrong type past a filter. These files let you see whether your system looks at the content and not just the name. A strong system detects the mismatch and refuses the file, or at least stores it with the correct type. Try each one in your upload form, media library and preview, and watch which checks catch them.