How to trigger your application while downloading file from wap browser(symbian)?

To achieve this,you have to follow three steps:
1.Write a recognizer.You have to specify MIME-TYPE  for you file.
    Refer to  http://www.forum.nokia.com/main/1,6566,040,00.html?fsrParam=3-3-/main/0,,1_32_30,00.html&fileID=2765 for an example.    

2.Modify your aif file to register your app.
   CHARACTER_SET UTF8
   RESOURCE AIF_DATA
    {
        ......
         embeddability=KAppEmbeddable;//KAppNotEmbeddable;
         newfile=KAppSupportsNewFile;//KAppDoesNotSupportNewFile;
         datatype_list=
        {
               DATATYPE { priority=EDataTypePriorityNormal; type="application/x-mm-xxx";}
         };
    }

3.Register your mime-type in wap server

Refer nokia Document Handler for further information