Fix preview of symbolic links (#739)

bin/preview.rb

@@ -33,7 +33,7 @@ unless File.readable? path
   exit 1
 end
 
-if `file --mime "#{file}"` =~ /binary/
+if `file --dereference --mime "#{file}"` =~ /binary/
   puts "#{file} is a binary file"
   exit 0
 end

bin/preview.sh

@@ -22,7 +22,7 @@ if [ ! -r "$FILE" ]; then
   exit 1
 fi
 
-if [[ "$(file --mime "$FILE")" =~ binary ]]; then
+if [[ "$(file --dereference --mime "$FILE")" =~ binary ]]; then
   echo "$1 is a binary file"
   exit 0
 fi