Solution 1
Define SelectCommand in class where you have 'attachment_id', 'attachment_name', 'attachment_price' properties.
<telerik:RadButton Content="Details" Name="btnDetails" Command="{Binding SelectCommand}" IsEnabled="True" CommandParameter="{Binding}"/>
Define SelectCommand execution in parent who is holding above collection. Pass the parent SelectCommand to individual object of above collection.
In your command execution, assign data you are receiving through CommandParameter binding as DataContext to Parent of your other TextBoxes.
Note: I am assuming you are following MVVM pattern here.
solution 2
Extract CommandParemeter in btnDetails_Click event handler from sender by casting it to RadButton.
Pass this CommandParemeter as DataContext to control who is hosting all other textboxes.