Non-static method common\components\DirectoryHelpers::mimeType() should not be called statically

totally php newbie here. need help about our library program here. i try to upload our book cover to our webserver, but get this error instead. the problem lines here

got this task to connecting our offline library sytem to webserver ( make it online ) this is what its looks like from the browser

The following code:

$mimetype=DirectoryHelpers::mimeType($files_uploaded);

Change as follows:

$var = new DirectoryHelpers();
$mimetype = $var->mimeType($files_uploaded);

Or as a shorthand:

$mimetype = (new DirectoryHelpers)->mimeType($files_uploaded);

In general: It depends on your needs. If you do not need to instantiate a class, you can change your library function to static. treat static functions like constants, (can not have objects)
Example here:

class DirectoryHelpers {
    public static function mimeType() {
        // code ...
    }
}