Fix for bug 2467152 files with spaces fail to open.

from http://code.google.com/p/android/issues/detail?id=4638

If you try to use a WebView to open an image (of any type)
that is stored in your project's assets/ directory the image
will be written to the screen as text (instead of drawn as
an image) if the filename contains a space.

The problem stems from MimeTypeMap, which determines the
file type from the url.  Spaces, represented as '%20'
are not included in the list of acceptable characters for
an image file name, causing the image to be treated as plain
text.  I am remedying this by adding '%' to the list.

Change-Id: I29e3da57f3cdaa63ed60b1e6977ba62a0dd108e5