M4R is a filename extension for AAC format, which is developed by Apple Inc. It is typically used to save custom iPhone ringtones from Apple iTunes. M4R can only be created from iTunes purchased songs and is usually DRM protected. It can open on iTunes, VLC Media Player, Apple QuickTime Player, and more.
According to YouTube, MP4 video format with AAC audio codec and H.264 video codec is most recommended. However, this platform is also compatible with formats such as MOV, AVI, WebM, FLV, 3GPP, WMV, and more. Also, keep in mind that the maximum file size for YouTube is 128GB or 12 hours of playtime. So, you can use a video editor like Wondershare UniConverter to compress before uploading videos to YouTube.
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