Hello! I encountered the same Problem using Joomla 3.5
I tried “Standard Search” and “Smart Search” module.
Neither could find the filenames.
Searching in article text works just fine.
I used the “Attachements Plugin” from Jonathan Cameron to add files to articles, for the old Version of the Page (Joomla 2.5). All files showed up when using the “Standard Search”.
I want to get rid of the need to manualy upload files through the web interface.
But loosing the ability to search for Filenames is to big a drawback.
Hope you will finde a solution.