This code places an Elementary file selector button widget on a window, along with some other checkboxes and a text entry. Those are there just as knobs on the file selector button's state and to display information from it.

Here's how we instantiate it:

   ic = elm_icon_add(win);
   elm_icon_standard_set(ic, "file");
   evas_object_size_hint_aspect_set(ic, EVAS_ASPECT_CONTROL_VERTICAL, 1, 1);

   /* file selector button */
   fs_bt = elm_fileselector_button_add(win);
   elm_fileselector_path_set(fs_bt, "/tmp");
   elm_object_text_set(fs_bt, "Select a file");
   elm_object_part_content_set(fs_bt, "icon", ic);

   elm_box_pack_end(vbox, fs_bt);
   evas_object_show(fs_bt);

Note that we set on it both icon and label decorations. It's set to list the contents of the "/tmp" directory, too, with elm_fileselector_button_path_set(). What follows are checkboxes to exercise some of its API functions:

   ck = elm_check_add(win);
   elm_object_text_set(ck, "editable selection");
   elm_check_state_set(ck, elm_fileselector_is_save_get(fs_bt));
   evas_object_smart_callback_add(ck, "changed", _current_sel_toggle, fs_bt);
   elm_box_pack_end(hbox, ck);
   evas_object_show(ck);

   ck = elm_check_add(win);
   elm_object_text_set(ck, "\"inwin\" mode");
   elm_check_state_set(ck, elm_fileselector_button_inwin_mode_get(fs_bt));
   evas_object_smart_callback_add(ck, "changed", _inwin_mode_toggle, fs_bt);
   elm_box_pack_end(hbox, ck);
   evas_object_show(ck);

   ck = elm_check_add(win);
   elm_object_text_set(ck, "folders only");
   elm_check_state_set(ck, elm_fileselector_folder_only_get(fs_bt));
   evas_object_smart_callback_add(ck, "changed", _folder_only_toggle, fs_bt);
   elm_box_pack_end(hbox, ck);
   evas_object_show(ck);

   ck = elm_check_add(win);
   elm_object_text_set(ck, "expandable");
   elm_check_state_set(ck, elm_fileselector_expandable_get(fs_bt));
   evas_object_smart_callback_add(ck, "changed", _expandable_toggle, fs_bt);
   elm_box_pack_end(hbox, ck);
   evas_object_show(ck);

   lb = elm_label_add(win);
   elm_object_text_set(lb, "Last selection:");
   elm_box_pack_end(vbox, lb);
   evas_object_show(lb);

   en = elm_entry_add(win);
   elm_entry_line_wrap_set(en, EINA_FALSE);
   elm_entry_editable_set(en, EINA_FALSE);
   evas_object_smart_callback_add(fs_bt, "file,chosen", _file_chosen, en);
   elm_box_pack_end(vbox, en);
   evas_object_show(en);

The checkboxes will toggle whether the file selector button's internal file selector:

must have an editable text entry for file names (thus, be in "save dialog mode")
is to be raised as an "inner window" (note it's the default behavior) or as a dedicated window
is to populate its view with folders only
is to expand its folders, in its view, in place, and not repainting it entirely just with the contents of a sole directory.

The entry labeled "Last selection" will exercise the "file,chosen" smart event coming from the file selector button:

static void /* hook on the sole smart callback */
_file_chosen(void *data, Evas_Object *obj EINA_UNUSED, void *event_info)
{
   Evas_Object *entry = data;
   const char *file = event_info;
   if (file)
     {
        elm_object_text_set(entry, file);
        printf("File chosen: %s\n", file);
     }
   else
     printf("File selection canceled.\n");
}

/* toggle inwin mode */

Whenever you dismiss or acknowledges the file selector, after it's raised, the event_info string will contain the last selection on it (if any was made).

See the full source code for this example.