using python-magic to get file mime type

tower: https://tower.im/projects/d4e0ff64158645f28d99a9ca72d0c27b/todos/d7a07b7339fa468ea8f074799abfb28a/

Change-Id: I93630b10f460038a1590958db0ee48226cd3049c
bugfix: open opengram clears playlist

tower: https://tower.im/projects/d4e0ff64158645f28d99a9ca72d0c27b/todos/323b66e30a5c405091c0400d91d4894f/
tower: https://tower.im/projects/d4e0ff64158645f28d99a9ca72d0c27b/todos/d57a8d0396d54f76837474934770bdae/

Change-Id: Ib1e19391702e4c5288820672e99d667f1f27565f