Currently this attribute has no effect.
At the first we thought that it should be binding where uploaded file's content type should be stored.
But at the moment this idea have no implementation in component spec.
If you think that this thing can be carefull we can provide implementation in the moment.
About acceptedTypes attribute:
File extensions should be separated by , or | or ; symbols.
This attribute does not effcet on the browser's filter.
Component just ignores adding files with different extensions.