You added a space just after track number, is this intentional ?


I would modify `picard/ui/options/renaming.py` this way:
```python
_DEFAULT_FILE_NAMING_FORMAT = "$if2(%albumartist%,%artist%)/$if($ne(%albumartist%,),%album%/)$if($gt(%totaldiscs%,1),%discnumber%-,)$num(%tracknumber%,2) $if(%_multiartist%,%artist% - ,)%title%"
```

```python
        config.TextOption("setting", "file_naming_format",
                          _DEFAULT_FILE_NAMING_FORMAT),
```

And i would add a test in `test/test_script.py`:
```python
from picard.ui.options.renaming import _DEFAULT_FILE_NAMING_FORMAT
```
```python
    def test_default_filenaming(self):
        context = Metadata()
        context['albumartist'] = u'albumartist'
        context['artist'] = u'artist'
        context['album'] = u'album'
        context['totaldiscs'] = 2
        context['discnumber'] = 1
        context['tracknumber'] = 8
        context['~multiartist'] = '1'
        context['title'] = u'title'
        result = self.parser.eval(_DEFAULT_FILE_NAMING_FORMAT, context)
        self.assertEqual(result, u'albumartist/album/1-08 artist - title')
        del context['~multiartist']
        result = self.parser.eval(_DEFAULT_FILE_NAMING_FORMAT, context)
        self.assertEqual(result, u'albumartist/album/1-08 title')
```

Of course, more various combinations can (and should) be tested.

What do you think about it ?