Adding some features on attachment page of any server

[GautamSYadav]

Is it possible to capture the events on a web page which is coming from any server (like yahoo, rediff or www.webdeveloper.com). for example when we attach a file while sending an attachment with yahoo mail, we click on "browse" button to open the system Open File Dialog box.then we select the file and attach it. Now if i want to do some custom action on this click event then is it possible with java script or not ?

Please be sure that I m talking about the page which is coming from the server and want to do some custom action on it as described above.

who is responsible to launch the "Choose File" Dialog box by clicking on Browse button to attach a file.. is there any javascript or it is windows responsiblity ?

Can we display a custom dialog box (instead of Choose File dialog box) when we click on on Browse button ?

Thanks in advance
Gautam