Bards in high-level games

[Azoriel]

Bard songs are mind affecting. There is no case to be made, if you are immune to mind affecting you are immune to them unless undead+requiem etc.

Your statement is incorrect.  Review the description of the Bardic Music ability: while (most) specific functions of the Bardic Music ability are indeed mind-affecting, Bardic Music as a whole does not bear that descriptor (only the specifics functions).  In particular, Countersong and Song of Freedom aren't mind-affecting at all.  (And it wouldn't make sense for them to be mind-affecting either; one involves nullifying sonic attacks, and the other breaks magical curses.)

Given that Dragonfire Inspiration doesn't have the mind-affecting descriptor written into the ability (unlike Fascinate and all those other abilities), the letter of the rules certainly don't support the assumption that it is mind-affecting either.  Furthermore, from a logical standpoint, what exactly is "mind-affecting" about magical fire shooting from other people's weapons?  Neither the letter nor the spirit of the rules support your statement in regards to DFI.

[X-Codes]

When you use your bardic music to inspire courage, you can choose to imbue your allies with dragonfire.   This choice is made when first activating the ability, and the choice applies to all allies affected.

Inspire Courage is Mind-Affecting, ergo Dragonfire Inspiration cannot affect allies immune to Mind-Affecting.

[Sleepyphoenixx]

If the only bardic music you want to use is DFI there's no reason to take more than one level of bard.
I did, and i'd like to actually use my abilities, so DFI being mind-affecting by association with IC or not doesn't really matter all that much to me.

[Mortadelas]

It is called DF "Inspiration".
Also
"When you use your bardic music to inspire courage, you can choose to imbue your allies with dragonfire."
It is not even a diferent ability, it is like the 3 ways to cast Telekinesis etc. Effectively you still Inspire Courage and choose a different effect.
Having said that, you are right, it doesn't make sence. However it is unclear enough to still be under DM interpretation. I'd more easily claim that a +4 int is mind affecting than dragonfire inspiration but I'm not the OPs DM.