Fix media.save throws an error in Umbraco 7.0.x

See http://issues.umbraco.org/issue/U4-3928 width and Height properties
are assigned on the file media types.  This was because of always
assigning a value to
content.getProperty(uploadFieldConfigNode.WidthFieldAlias).Value

Added an if statement to make sure the media item is an image when
assigning those values.