BOF, offset 4: 'ftyp' specifying 'M4A ' as the major brand. Note that not all AAC audio files using the .m4a extension will carry this signature. Files produced by non-Apple toolchains may use a generic major brand such as 'mp42' or 'isom' despite containing equivalent audio content. For now any such files will continue to identify as fmt/199 MPEG-4 Media
Details: File format summary
: detailed report on file format 
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