Extracts a file uploaded by means of the "file selection" field fieldname and stores it under savepath.
May be the name of a plain file or an existing directory in which the file is to be created under its original name. Any existing file under the same name will be overwritten.
There seems to be no other way to find out the original filename but to store the file using a directory as savepath and check for the basename it was stored under.
The returned array contains the following fields: