Fix inline media preview height

When using JFormFieldMedia with `preview="true"`, preview height is hardcoded to 18px by bootstrap.

`preview="tooltip"` is not affected.

Issue Tracker: [#31073](http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31073)

___
*Before patch:*
![joomla_patch_formfieldmedia_before](https://f.cloud.github.com/assets/612953/600435/7c8c0c6e-cc67-11e2-8f5d-5dd7e3af11b8.png)

*After patch:*
![joomla_patch_formfieldmedia_after](https://f.cloud.github.com/assets/612953/600436/80a4d95c-cc67-11e2-9ea4-b2de19b04923.png)