MDL-55324 filepicker: Explicitly set callback from showFilePicker

Prior to this patch the callback passed to showFilePicker was saved
and reused for subsequent calls. This doesn't allow the use of a function
for generating the callback.

This patch explicitly sets the callback each time.

repository/filepicker.js

@@ -498,6 +498,7 @@ M.core_filepicker.show = function(Y, options) {
     if (!M.core_filepicker.instances[options.client_id]) {
         M.core_filepicker.init(Y, options);
     }
+    M.core_filepicker.instances[options.client_id].options.formcallback = options.formcallback;
     M.core_filepicker.instances[options.client_id].show();
 };