Do extract metadata in separate task.

Unicode filenames trigger an error in:
  File "./libxmp/exempi.py", line 459, in files_open
    EXEMPI.xmp_files_open(xfptr, filename.encode('utf-8'), options)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 77: ordinal not in range(128)