This is interesting (maybe)...
After 12 BSODs during installation from the Cata DVD, I finally got to 18.104.22.16811. Since I've been through this a bunch of times with varying degrees of success, I thought I might just see what all the fuss is about with the MFIL files.
I grabbed the 22.214.171.12495 MFIL and put together the URL to download the following:
Once downloaded, I extracted it using Ladik's MPQ editor. This file contains Launcher.exe, WoW.exe and other files you'd find in the root WoW directory. Here's the interesting part though... notice above it says fileversion=15050 which indicates that this MPQ file is current up to 126.96.36.19995? Well guess what version the WoW.exe is in that file? Right, it's 188.8.131.5211 even though the MFIL is for 184.108.40.20695.
Now not every patch has an MFIL and also some of the MFIL have this: 12911.streaming instead of 15050.direct. The MFIL started using the "direct" label at patch wow-220.127.116.1150-to-18.104.22.16811 and interestingly enough, this is the first patch where you could patch any version of WoW without needing previous patches.
Just for kicks, I decided to check base-Win.MPQ from an older, streaming version:
Not surprisingly, this file comes up as 404 on all four of blizzard's distribution servers if you use 12911.streaming in the URL but changing it to 12911.direct nets you a healthy MPQ file with identical files as the ones with fileversion=15050. In fact, it's the same MPQ. The filesize is equal to the size from the 15050 URL even though the size for the 12911 URL clearly indicates a different file.
A theory I'm testing now is to direct download all the MPQ files in the 15595 MFIL file because it appears that the streaming versions of the files have been removed.
Well after pre-allocating all the files, it only comes out to 14 GB so I doubt that direct downloading these is the way to go. The downloader must be doing some sort of "diffing" but my skills with OllyDbg are pretty crap so I couldn't figure out what it's really doing.
It's interesting that in all the MFIL files, locale_enCN and locale_zhCN have an alternate.mpq which is not available to any other locale. I Wonder what's in it considering it's over 1GB in size.