[underdeserver]

In general you're right, but I can't think of a single local use for identifying file types by a human on a laptop - at least, one with scale where this matters. It's all going to be SaaS services where people upload stuff.

[prmph]

We are building a data analysis tool with great UX, where users select data files, which are then parsed and uploaded to S3 directly, on their client machines. The server only takes over after this step.

Since the data files can be large, this approach bypasses having to trnasfer the file twice, first to the server, and then to S3 after parsing.

[DontSignAnytng]

This dont sound like very common scenario.