Minimum version: Midgard 1.4 (Bifrost)
Opens a filehandle to the attachment with id id.
mode may be any of the following:
'r' - Open for reading only; place the file pointer at the beginning of the file.
'r+' - Open for reading and writing; place the file pointer at the beginning of the file.
'w' - Open for writing only; place the file pointer at the beginning of the file and truncate the file to zero length. If the file does not exist, attempt to create it.
'w+' - Open for reading and writing; place the file pointer at the beginning of the file and truncate the file to zero length. If the file does not exist, attempt to create it.
'a' - Open for writing only; place the file pointer at the end of the file. If the file does not exist, attempt to create it.
'a+' - Open for reading and writing; place the file pointer at the end of the file. If the file does not exist, attempt to create it.
The optional parameter mode will default to "w" if omitted. Returned value is a file pointer which can be used with the usual PHP file functions. Use fclose() on the filehandle when done.
Returns a filehandle if successfull. Returns false on failure.