Rudimentary mimetype detection, from first few bytes, in C.

mime.c

#include "stdio.h"

// Get more from https://en.wikipedia.org/wiki/List_of_file_signatures

const char* detectMime(unsigned char* bytes) {
  if (bytes[0] == 0xFF && bytes[1] == 0xD8 && bytes[2] == 0xFF) {
    return "image/jpeg";
  }
  if (bytes[0] == 0x89 && bytes[1] == 0x50 && bytes[2] == 0x4E) {
    return "image/png";
  }
  if (bytes[0] == 0x4F && bytes[1] == 0x67 && bytes[2] == 0x67) {
    return "audio/ogg";
  }
  if (bytes[0] == 0x52 && bytes[1] == 0x49 && bytes[2] == 0x46 && bytes[8] == 0x41) {
    return "audio/wav";
  }
  if ((bytes[0] == 0xFF && (bytes[1] == 0xFB || bytes[1] == 0xF3 || bytes[1] == 0xF2)) || (bytes[0] == 0x49 && bytes[0] == 0x44 && bytes[0] == 0x33)) {
    return "audio/mp3";
  }
  return "application/octet-stream";
} 


int main(int argc, char *argv[]) {
  if (argc < 2) {
    fprintf(stderr, "Usage: %s <file>\n", argv[0]);
    return 1;
  }

  char b[9];
  FILE *fptr;
  fptr = fopen(argv[1], "r");
  fgets(b, 9, fptr);
  fclose(fptr);
  printf("%s\n", detectMime(b));

  return 0;
}