Update file_list image styling to only be applied to file_list type
Image preview size for file_list field

Added option to specify image preview size for file_list field. Accepts
standard WordPress size i.e. either a string keyword (preregistered
image size) or a 2-item array representing width and height in pixels,
e.g. array(100, 100).
Merge remote-tracking branch 'upstream/trunk' into trunk
Merge pull request #471 from IgorCode/trunk

Image preview size for file_list field. Props @IgorCode.