Finder window on top of full-screen safari?
Rather clunky, but possible...


Go back to Finder, select &amp; grab your files; keep hold of them.  
either...


Use your Show Dashboard key command &amp; hover your cursor [still holding the files] over the Safari Space until it pops to the front, or  
Use  Ctrl ⌃   num  to go to the last Space before your fullscreen app, then  Ctrl ⌃   →  to get to fullscreen.  

Drop the files.


This is always going to be hindered by fullscreen apps being to the right of any &amp; all numbered Spaces.

You may need to enable the Ctrl/key functionality first in System Prefs/Keyboard/Shortcuts/Mission Control 



Only alternative I can think of is use the Open dialog, or as suggested elsewhere, don't fullscreen, just maximise.