oracle - how to show zero on the left before decimal point when using round()? -

i used round() function show file size in byte mb.

the data following-

file_size ---------- 2463

i used convert-

round(document.file_size / (1024*1024), 3) file_size

now data showing this-

file_size ---------- .002

but, want this-

file_size ---------- 0.002

just left zero. how can have this?

try to_char(round(document.file_size / (1024*1024), 3), 990.000) file_size

Alconcel