MP4 is the safest upload target
When a video will not upload, MP4 is usually the first format to try. It works across most social platforms, phones, browsers, and editors.
When WebM helps
WebM can be useful for browser-first video and web publishing. If the destination is a social platform or generic upload form, MP4 is usually safer.
Review before publishing
Check audio sync, duration, aspect ratio, and playback after conversion. A smaller file is only useful if it still plays correctly.