xpcse client installation:

   1. copy "xpcse" and "run-mailcap" to a directory included in $PATH
      (if you install PApp, it will be copied automatically to $Config{sitebin}
       so this step should not be needed in this case)
   2. Add the "xpcse" helper application to your browser.
      Mozilla:
        Edit->Preferences->Helper Applications,
          Name: xpcse
          Extension: xpcse
          MIME-Type: application/x-xpcse
      Netscape:
        Add
          application/x-xpcse;xpcse "%s"
        to ~/.mailcap
   3. create a ~/mailcap.xpcse (private) or "/etc/mailcap.xpcse" (system-wide)
      to fork applications that have a specific content-type. xpcse first
      reads your .mailcap.xcpse and then your .mailcap.
      Example (edit-method is used)
      
      image/gif; /usr/bin/gimp "%s"; edit=/usr/bin/gimp "%s"

It should work now.