How can I check if a file is mp3 or image file?

You can identify image files using getimagesize.

To find out more about MP3 and other audio/video files, I have been recommended php-mp4info getID3().

Native ways to get the mimetype:

For PHP < 5.3 use mime_content_type()
For PHP >= 5.3 use finfo_fopen()

Alternatives to get the MimeType are exif_imagetype and getimagesize, but these rely on having the appropriate libs installed. In addition, they will likely just return image mimetypes, instead of the whole list given in magic.mime.

If you don't want to bother about what is available on your system, just wrap all four functions into a proxy method that delegates the function call to whatever is available, e.g.

function getMimeType($filename)
{
    $mimetype = false;
    if(function_exists('finfo_fopen')) {
        // open with FileInfo
    } elseif(function_exists('getimagesize')) {
        // open with GD
    } elseif(function_exists('exif_imagetype')) {
       // open with EXIF
    } elseif(function_exists('mime_content_type')) {
       $mimetype = mime_content_type($filename);
    }
    return $mimetype;
}

To find the mime type of a file I use the following wrapper function:

function Mime($path)
{
    $result = false;

    if (is_file($path) === true)
    {
        if (function_exists('finfo_open') === true)
        {
            $finfo = finfo_open(FILEINFO_MIME_TYPE);

            if (is_resource($finfo) === true)
            {
                $result = finfo_file($finfo, $path);
            }

            finfo_close($finfo);
        }

        else if (function_exists('mime_content_type') === true)
        {
            $result = preg_replace('~^(.+);.*$~', '$1', mime_content_type($path));
        }

        else if (function_exists('exif_imagetype') === true)
        {
            $result = image_type_to_mime_type(exif_imagetype($path));
        }
    }

    return $result;
}