When reading EXIF data for large tiff files, our optimistic
file prefix sometimes isn't enough and we need to pass in
the whole file. We already did this in several places (image
decoding and indexing), this change adds it for finding the
modtime (for which we try to use EXIF data when
available). It pulls common functionality out into a
separate func and changes the existing uses of this pattern
to make use of the func.
Change-Id: I2b786775f168f47f46fb5ac707e3744991139a21