The /r/askhistorians subreddit has given me an inordinate amount of pleasure over the past decade. This is a heavily moderated forum for academic historians to write well-sourced, long form answers to specific questions about their expertise. Where you can get a thoughtful answer from one of the few dozen people on Earth knowledgeable enough to supply it.
For the uninitiated, a great place to start is the AskHistorians Wiki, which has links to many frequently asked questions such as: “What did people think about static electricity shocks before electricity was discovered?”, or “How would medieval fighters/warriors/military recognize friend from foe?”, or the particularly insightful and personal favorite “Holocaust Denial and how to combat it”.
I was recently able to answer a question on “sandpaper and files in the Middle Ages”, and it was truly an honor to meet the rigorous bar for commenting in this forum, and to give back a small nugget of information to the community. I recommend clicking the above link and reading all of the wonderful replies in the thread… I'm not a historian, and there is fantastic historical context for my more technical discussion.
“I can imagine the process for making a rough file. But did Europeans in the middle ages have a process for making a file with an emeryboard texture?”
I'm not a historian, but I am an engineer working in advanced manufacturing of metal components with a graduate degree (and professional experience) around metallurgy.
I can't answer about sandpaper here, but I can tell you a little bit about files.
First, why does a file even work? Why does a file cut a part and not the other way around? It's not just about the rough surface— try rubbing a copper file on a steel bar and see how far you get. Files (and material removal processes generally) depend on the relative hardness of the two materials, with the harder material removing bits of the softer material. This is highly simplified but we'll leave the technical aside there since this is /r/askhistorians and not /r/askscience.
The question you're really asking is about the hardening of steel and its application in tool making. The earliest written description of these processes was compiled around 1100-1120 by Theophilus in his treatise "On Divers Arts" [1], where he writes fairly detailed instructions on a variety of artistic and industrial techniques (painting, glassblowing, metalworking, etc.). In it he describes case hardening a part via carburization, which is the process of adding extra carbon to the surface of a low-carbon steel, allowing the surface to become hard and brittle while the core remains soft and ductile. [Edit: see /u/xeimevta's response below for excellent context and quotes from Theophilus]
The basic process is to cover a low-carbon steel (read: wrought iron) part in carbon-containing material (charcoal, burned bone, old fat), heat it in a low-oxygen environment (achieved by sealing in a clay envelope), and then quench the hot part in liquid to rapidly cool the surface. Carbon atoms diffuse into the surface of the part, making a thin layer of high-carbon steel, which is then hardened during the rapid quench. Smiths and artisans of the middle ages understood how to carburize but did not understand the process mechanistically, making it a bit of a magical art. Myths abounded in this environment, as described by Theophilus, who is adamant that the best quenching liquid is "the urine of a red-haired boy".
So to directly answer your question about file making in any period up to the Renaissance:
Roughly form the file shape from raw wrought iron, using a combination of forging and grinding on stone wheels.
"Strip" the surfaces of the file to be completely flat
Form the teeth using a chisel on each cutting surface of the file (the chisels are made from the same steel and hardened prior to this process)
Harden the fully formed files via carburization, quench in liquid, and then (optionally) temper to the desired final hardness.
I highly recommend checking out Clickspring's Youtube channel, but specifically the two videos I'll link below. For the past 4-5 years he's been working on the enormous project of reverse engineering the Antikythera Mechanism from detailed CT scans of its internal mechanisms, and then manufacturing one using as many period-accurate techniques as possible. As part of this project he has performed original research on tools and techniques, and has co-authored a paper currently under peer review. These should illustrate the answer to your question on files.
Antikythera Fragment #3 - Ancient Tool Technology - Hand Cut Precision Files
Antikythera Fragment #4 - Ancient Tool Technology - The First Hardened Steel
I know that /r/AskHistorians isn't big on Youtube sources, but in this case I feel it should be allowed, it's shockingly on topic.
I answered a few followup questions; the most significant and fun to write was about medieval quenchants like blood and urine:
“Myths abounded in this environment, as described by Theophilus, who is adamant that the best quenching liquid is “the urine of a red-haired boy.”
Well? Is it?”
I will refer here to the excellently titled Beer, Blood and Urine - Mythological Quenchants of Ancient Blacksmiths, a conference paper available to read in full at that Researchgate link. It's a bit tongue in cheek, but on reading it's honestly a fantastic academic paper.
Some choice quotes from the historical literature:
Tools are also given a harder tempering in the urine of a small, red-headed boy than in ordinary water. (Theophilus, 1125)
Take clarified honey, fresh urine of a he-goat, alum, borax, olive oil, and salt; mix everything well together and quench therein. (Smith, Biringuccio, Agricola, et. al.)
Giambattista della Porta demonstrates an understanding of the underlying chemical mechanisms responsible for quenching outcomes in the 16th century:
“If you quench red hot iron in distilled vinegar, it will grow hard. The same will happen, if you do it into distilled urine, by reason of the salt it contains in it. f you temper it with dew, that in the month of May is found on vetches leaves, it will grow most hard. For what is collected above them, is salt, as I taught elsewhere out of Theophrastus. Vinegar, in which Salt Ammoniac is dissolved, will make a most strong temper. But if you temper Iron with Salt of Urine and saltpeter dissolved in water, it will be very hard. Or if you powder Saltpeter and Salt Ammoniac, and shut them up in a glass vessel with a long neck, in dung, or moist places, till they resolve into water, and quench the red hot Iron in the water, you shall do better. Also iron dipped into a Liquor of Quicklime, and Salt of Soda purified with a Sponge, will become extreme hard. All these are excellent things, and will do the work.”
And on the experimenter’s procedure:
[...] it was decided to use a heavy hopped beer. Once selected, after a thorough tasting process (quality control purposes)
[...]
Blood was more of a technical and political challenge.
But to your actual question— unsurprisingly, urine does change the quench dynamics. I say unsurprisingly because brine quenching is a standard commercial process, and urine is basically a type of brine with dissolved sodium chloride and potassium chloride. In the author's words:
the synthetic urine is a very fast quenchant and greatly exceeds the cooling rate of water. This is thought to be caused by the salts precipitating on the surface of the probe, resulting in nucleation sites for nucleate boiling to occur. This cooling curve is similar to commercial quenchants containing inorganic salts.
[...]
The two fastest quenchants, water and synthetic urine yielded a predominately martensitic structure. However, because of the speed of the quenchant, it is likely to result in cracking of the knives, especially if urine is used.
No word on the effects of urine specifically from a red-haired boy.