Detecting file MIME in C

I have files with wrong extensions, and try to find the correct MIME in a C script.

For a PDF file with txt extension, magic (#include <magic.h>)

  const char *mime;
  magic_t magic;
  magic = magic_open(MAGIC_MIME_TYPE); 
  magic_load(magic, NULL);
  magic_compile(magic, NULL);
  mime = magic_file(magic, filename);
  printf("%s\n", mime);
  magic_close(magic);

returned

application/octet-stream

which is not very helpful.

GIO 2.0 (#include <gio/gio.h>)

  char *content_type = g_content_type_guess (file_name, NULL, 0, &is_certain);

  if (content_type != NULL)
    {
      char *mime_type = g_content_type_get_mime_type (content_type);

      g_print ("Content type for file '%s': %s (certain: %s)\n"
               "MIME type for content type: %s\n",
               file_name,
               content_type,
               is_certain ? "yes" : "no",
               mime_type);

      g_free (mime_type);
    }

returned

Content type for file 'test.txt': text/plain (certain: no)
MIME type for content type: text/plain

However, file command in Linux returns the correct MIME:

file test.txt
test.txt: PDF document, version 1.6

This should not be the expected behaviors of these well-established libraries in C. What do I do wrong?