Creating files

i have
<%= file_field_tag “logo” %> using that i upload image files.,.In
controller how can i write this to a folder with a given fixed name…For
example suppose uploaded file original name is abc.png,(or abc.jpeg )but
in controller i have to name it to logo.png or logo.jpeg and write it to
a folder /opt/upload/logo.png (or /opt/upload/logo.jpeg) That means
if the file there already exists it shud overwrite also …How can i do
this .Please help

Thanks in advance