//echo -a $tempfn
It does not create the disk filename, but offers the string as a "safe" filename that can be used without it already existing as a disk file.
The string returned is $mircdir $+ mirc $+ $rand(0,999998) $+ .tm_
Note: The filename mask is the same used internally by mIRC for disk write fuctions, such as in the /write command. If a file you /write to disappears, it's possible that it exists in the same folder with a filename matching the wildcard mirc*.tm_.
Added: mIRC v7.46
Added on: 01 Aug 2016
Note: Unless otherwise stated, this was the date of original functionality.
Further enhancements may have been made in later versions.