It is Wu-ftpd however since that is the most common ftp server on the net that's not too much of a suprise.
Anyway, I just checked RFC's and the behavior of the server with respect to the i.e. download logic and I see no problem from the server end:
Here is the sequence of commands that i.e. uses when trying to get the file:
Aug 26 11:04:31 ftp2 wu-ftpd[30089]: connection from 192.146.151.180 [192.146.151.180]
Aug 26 11:04:31 ftp2 wu-ftpd[30089]: VirtualFTP Connect to: ftp.jriver.com [198.73.82.66]
Aug 26 11:04:31 ftp2 wu-ftpd[30089]: USER anonymous
Aug 26 11:04:31 ftp2 wu-ftpd[30089]: PASS IEUser@
Aug 26 11:04:31 ftp2 wu-ftpd[30089]: ANONYMOUS FTP LOGIN FROM 192.146.151.180 [192.146.151.180], IEUser@
Aug 26 11:04:31 ftp2 wu-ftpd[30089]: TYPE Image
Aug 26 11:04:31 ftp2 wu-ftpd[30089]: PASV
Aug 26 11:04:31 ftp2 wu-ftpd[30089]: SIZE /pub/downloads/music/MediaCenter110068.exe
Aug 26 11:04:31 ftp2 wu-ftpd[30089]: RETR /pub/downloads/music/MediaCenter110068.exe
Notice that the only thing i.e. does to check the file is the size command.
Now if I run the size myself on a raw connection I get back the correct size:
SIZE /pub/downloads/music/MediaCenter110068.exe
213 18341332
Now, i.e. COULD do a stat command:
STAT /pub/downloads/music/MediaCenter110068.exe
213-status of /pub/downloads/music/MediaCenter110068.exe:
-rw-r--r-- 1 210 18341332 Aug 25 22:11 /pub/downloads/music/MediaCenter110068.exe
To see that the date has changed, but it doesn't.