This example assumes that you have a directory object set up where the files are stored. In this example the following object is used:

CREATE DIRECTORY FILEUPLOADS AS '/home_new/dmt/public_html/fileuploads';

Use the following function to load a file based on the filename parameter and return the BLOB reference for processing:

CREATE OR REPLACE FUNCTION loadBlobFromFile(p_file_name VARCHAR2) RETURN BLOB AS
  dest_loc  BLOB := empty_blob();
  src_loc   BFILE := BFILENAME('FILEUPLOADS', p_file_name);
BEGIN
  -- Open source binary file from OS
  DBMS_LOB.OPEN(src_loc, DBMS_LOB.LOB_READONLY);

  -- Create temporary LOB object
  DBMS_LOB.CREATETEMPORARY(
        lob_loc => dest_loc
      , cache   => true
      , dur     => dbms_lob.session
  );

  -- Open temporary lob
  DBMS_LOB.OPEN(dest_loc, DBMS_LOB.LOB_READWRITE);

  -- Load binary file into temporary LOB
  DBMS_LOB.LOADFROMFILE(
        dest_lob => dest_loc
      , src_lob  => src_loc
      , amount   => DBMS_LOB.getLength(src_loc));

  -- Close lob objects
  DBMS_LOB.CLOSE(dest_loc);
  DBMS_LOB.CLOSE(src_loc);

  -- Return temporary LOB object
  RETURN dest_loc;
END loadBlobFromFile;
/

Easy as that.