Diamond files tend to be very fine grained. You won't remove material very fast.
For working with unhardened steel, I would start with the first 2 classes of files below. I'm grouping them in a way that if you find one that's in a group, it should be pretty comparable to the others within that group, so you have some leeway. The ones in group 3 and 4 are more generic for more finishing work.
Group 1: Magicut, Multi-cut (Multi-kut), Chipbreaker:
These files are very aggressive. Made to remove materials fast but leave a pretty clean finish. Though technically single cut files, their design and coarse teeth cut more aggressively than any double cut bastard file. I would use these as the main heavy removal. Nicholson calls theirs Magicut, you'll want to find the ones made in America. The teeth pattern is a bit different than the next two in the group, but they're all designed with the same purpose in mind and the difference in the teeth pattern has more to do with keeping the teeth clear of filings. The best place to find this is eBay but be sure to notice whether or not it's the older American made one or the newer Mexican/Brazil made. If it doesn't say specifically, ask the seller before buying. Grainger's might have the older American made Magicuts as well. Just be sure to avoid the Mexican made ones.
Simonds calls their Multi-kut (some spell it Multi-cut). It's harder to find the American made Simonds Multi-kut, and Samraistuart says the Honduras one is good and they're easy to find on eBay or other google searches.
Pferd makes a design exactly like the Simonds Multi-kut, and calls it the Chipbreaker. I have a couple of them. Pferd is made in Germany and so far I find their files very good. I can actually get my hands on the 12" Pferd Chipbreaker very easily for less than $5 at a welding shop about an hour away from me. So if you want I can grab one for you at cost and shipping. Just PM me if you need me to do that.
Group 2: Long angle lathe file
This is really good for draw filing because of the cuts in the file are angled at 45 instead of 30. This means that when you turn the file to do draw filing, the angle remains at 45, while other files that angle becomes 120 (or 60 depending on how you look at it). The 120 degree is too close to parallel with your draw filing stroke, so you end up not removing material as efficiently and you're at higher risk of having the file cut grooves into the material.
As far as brand, I've only been able to come across Nicholson brand. Again, get only the older American made ones. Check eBay. Also check your local Grainger's. Ironically Grainger's has been trying to clear out the American made files and if you can find them there they're cheaper than the lesser quality Mexican made Nicholsons.
Group 3: Flat mill files
The ones in group 1 takes care of heavy removal, the mill file takes care of finer removal. Any brand as long as it's good condition old stock American made, or if new then Swiss, German, Swedish made.
Group 4: Half rounds, rounds
Depending on your needs for curved shapes. As with group 3, brand won't matter as much as country it's made in.
