Upload audio file "is required" every time

Hey, I just would like to create a file upload. With this form I just would like to upload audio files, so I implement the form like:

{!! Form::open(['route'=>'beats.store', 'method'=>'post', 'class'=>'form-horizontal', 'files'=>true]) !!}
            <div class="form-group{{ $errors->first('audio', ' has-error') }}">
    <label for="audio" class="col-sm-2 control-label">@lang('beats.form.audio')</label>

    <div class="col-sm-10">
        {!! Form::file('audio') !!}
        {!! $errors->first('audio', '<p class="text-danger">:message</p>') !!}
    </div>
</div>
// ...
{!! Form::close() !!}

The request-rules-method looks like:

public function rules()
    {
        Validator::extend('audio', function($attribute, $value, $parameters)
        {
            $allowed = array('audio/mpeg', 'application/ogg', 'audio/wave', 'audio/aiff');
            $mime = new MimeReader($value->getRealPath());
            return in_array($mime->get_type(), $allowed);
        });

        return [
            'name'=>'required',
            'bpm'=>'required|numeric',
            'artists'=>'required',
            'selled'=>'required_if:buyable,on',
            'contact'=>'required_if:buyable,on',
            'audio'=>'required|audio'
        ];
    }

An image-upload works - but every time I would like to upload an audio-file, it fails and says, "The audio field is required.". The mime type, which laravel detects, is: "application/octet stream" - on any audio-file.

Any ideas? :/