img: Explicitly detect format if needed

img: Explicitly detect format if needed

An empty coder name used to detect the format implicitly, but has been
interpreted as a literal part of the filename since ImageMagick 6.9.8-3.
In newer versions, there does not seem to be any way to indicate that
a filename containing ':' is to be taken literally without first
knowing the decoder to use.

Signed-off-by: Simon McVittie <smcv@debian.org>