toaster: open image files in binary mode when sending in response

toaster: open image files in binary mode when sending in response

The view code for downloading image files used the "r" flag
to read the file, then used the open file object to form the
HTTP response.

While this worked in Python 2, Python 3 appears to be more strict
about this sort of thing, and Django throws a UnicodeDecodeError
when a file opened this way is used in a response.

Open the file with the "b" flag (binary mode) so that Django can
correctly convert the binary file handle to an HTTP response.

Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>