How to display a field in a content type template?
As per my knowledge you can not use $content array in field.tpl.php.
You need to use $items.


  Available variables:

$items: An array of field values. Use render() to output them.
$label: The item label.
$label_hidden: Whether the label display is set to 'hidden'.
$classes: String of classes that can be used to style contextually through CSS.

  
  from preprocess functions. The default values can be one or more of
  the following:

field: The current template type, i.e., "theming hook".
field-name-[field_name]: The current field name.
field-type-[field_type]: The current field type. 
field-label-[label_display]: The current label position. 
$element['#object']: The entity to which the field is attached.
$element['#view_mode']: View mode, e.g. 'full', 'teaser'...
$element['#field_name']: The field name.
$element['#field_type']: The field type.
$element['#field_language']: The field language.
$element['#field_translatable']: Whether the field is translatable or not.
$element['#label_display']: Position of label display, inline, above, or hidden.
$field_name_css: The css-compatible field name.
$field_type_css: The css-compatible field type.
$classes_array: Array of html class attribute values