Get file field alt text value using entity_metadata_wrapper
The file object does not contain the image metadata (Title, Alt,..). You must load the parent node to which the image is attached.

For example to get the Title of the 3rd image in the field "field_image" you would use:

$obj = entity_metadata_wrapper('node', $nid);
$obj-&gt;field_image[3]-&gt;title = $data;


Note 1: you do not have to load the node object first - the $nid variable can be the node id (entity_id)

Note 2: You should use the dpm() function to inspect the entity instead of var_dump. If you want to see also values of properties in the inspected entity, you can use a custom function. Google _wrapper_debug function to get it.