Gallery2: fix NPE when you save edited picture in file explorer.

If view a picture from file explorer, the scheme of
the uri is "file", and it gets null when it gets mimeType
from the uri, a NullPointerException happens.

If the scheme of the uri is "file", we get mimeType
in another way.

CRs-Fixed: 522216

Change-Id: I7111a4e3fdb2d8c5eea7646d147d829de4ff7d33