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file: add (very) basic support for detecting mpeg4 formats

--- a/sys/src/cmd/file.c

+++ b/sys/src/cmd/file.c

@@ -152,6 +152,7 @@

 int	islimbo(void);

 int	istga(void);

 int	ismp3(void);

+int	ismp4(void);

 int	ismung(void);

 int	isp9bit(void);

 int	isp9font(void);

@@ -201,6 +202,7 @@

 	isface,		/* ascii face file */

 	istga,

 	ismp3,

+	ismp4,

 	/* last resorts */

 	ismung,		/* entropy compressed/encrypted */

@@ -1238,6 +1240,24 @@

 			return 1;

 		}

 		p++;

+	}

+	return 0;

+}

+

+int

+ismp4(void)

+{

+	if(nbuf <= 12)

+		return 0;

+	if(memcmp(&buf[4], "ftyp", 4) != 0)

+		return 0;

+	if(memcmp(&buf[8], "isom", 4) == 0){

+		print("%s\n", mime ? "video/mp4" : "mp4 video");

+		return 1;

+	}

+	if(memcmp(&buf[8], "M4A ", 4) == 0){

+		print("%s\n", mime ? "audio/m4a" : "m4a audio");

+		return 1;

 	}

 	return 0;

 }