Hello..
I'm not sure if this will help you or not, but this is what I have configured to show the file type icon for our files of a list.
Create an Image type column; for this example we'll call it Icon. For the List of values, enter the icon names and the icon locations. E.g.:
PDF|http://www.mywebsite.com/Portals/0/resources/images/homepage/gen_pdf_file_icon.png;Agenda|http://www.mywebsite.com/Portals/0/resources/images/homepage/agenda_icon.png;ZIP|http://www.mywebsite.com/Portals/0/resources/images/homepage/zip_folder_icon.png;NoPDF|http://www.mywebsite.com/Portals/0/resources/images/homepage/gen_pdf_nofile_icon.png;NoZip|http://www.mywebsite.com/Portals/0/resources/images/homepage/nozip_folder_icon.png
In essence, we have PDF, Agenda, ZIP, NoPDF and NoZIP icons. The end-users only see these names while they're filling out the form.
We then have a calculated column which takes our Icon field and another field and combines them. For this example, I'll call the calculated column Information, set it's data type to Text and set it's Expression to: "[Icon] + ' ' + [File Information]". The File Information column we're referencing here is just a simple HTML text box which we configure to link text to the file location.
I'm not sure if this is what you're looking for but hope it helps. -Jeff