Still-image formats
JPEG is efficient for photographs but uses lossy compression. PNG is lossless and supports transparency. WebP can encode still or animated images efficiently and is common in web delivery.
GIF is a special case
GIF supports frame-based animation but has a limited colour model. Many social platforms display content users call GIFs while actually delivering a looping video file because modern video compression is substantially more efficient.
MP4 is a container
An .mp4 file is a container that can hold video and audio streams. The codec inside matters: H.264 video and AAC audio remain widely compatible. A filename extension alone does not describe every technical property of a file.
Choose a format based on the job
Use JPEG for ordinary photographs when small files and broad compatibility matter. Use PNG for screenshots, line art or images that require lossless storage and transparency. WebP can be efficient for modern web imagery, but check compatibility with your target workflow.
MP4 is generally appropriate for broadly playable social-video downloads when the contained video and audio codecs are supported by the receiving device. The container extension alone does not tell you whether every player can decode the streams.
How to verify what you actually downloaded
Start with the file extension and MIME type, then inspect dimensions and duration in ContentDown’s local inspector. If you need detailed codec and bitrate information, use a trusted local media analysis utility; browser APIs do not consistently expose all encoded stream metadata.
A moving picture displayed as a GIF in an app may download as MP4. That is not necessarily a conversion error; the service may have supplied video rather than a classic GIF file.