Viewing of BFILEs seems to have stopped working in the latest version (6.0.2.880) - I'm sure that this used to work.
Running an SQL like:
select dbms_lob.fileexists(bfilename('&dir','&file')) fileexists
,dbms_lob.getlength(bfilename('&dir','&file')) filelength
,bfilename('&dir','&file')...