HEEELP - Visual Studio .net - Custom controls

Hi, i created a custom control with visual studio .net, which should read an file. the user who wants to use the control in his project should be able to choose in the properties window the file. Unfortunately I just found out how to write something in there (i already have the property, but as a string), but there should be something like the little button with the 3 point which helps you to select a file and which creates the working string itself, so that the user cant just write anything in. Hope you can help me...