Containers and codecs explained
Why an MP4 may still fail to preview or process.
An extension is not a codec
MP4, MOV, MKV and WebM describe containers that hold video, audio and other tracks. H.264, HEVC, VP9 and AV1 describe different video encodings. Two files ending in .mp4 may contain very different codecs. Changing the filename extension does not convert a video and can make the file misleading.
Preview support comes first
Gomav checks the source in your browser before processing. A file may be valid while still being unsupported by that browser. MP4 with H.264 video and AAC audio is a useful compatibility test. If the preview cannot load metadata, try converting the source in a trusted local editor rather than renaming it.
The output format is explicit
Trim and remove-audio produce H.264 MP4 files; frame extraction produces PNG. The trim output includes AAC audio only when the source has an audio stream. Gomav does not claim to preserve every subtitle, alternate audio track, chapter or embedded metadata field. Check the exported output against your intended use.
Limits are part of compatibility
The current input limit is 100 MB, 10 minutes and 1920 × 1080. These are upper acceptance limits, not a guarantee that every device can process every accepted file. Browser memory, codec complexity and battery-saving settings affect success. Keep the tab open while working and use a smaller source when processing fails.