If you explicitly requested your browser to create a file, then it could be any kind of data. If the web server triggered the download, your browser probably saw an HTTP header like "Content-type: application/octet-stream"

I will admit that there is an abstract concept of a "file" that exists outside the restricted realm of a "filesystem," but that's not amazon's target audience. Their target audience are programmers using APIs.

To a normal person, MySpreadsheet.xls is considered a file, no matter where it is stored. But S3 is not limited to this definition of file. An S3 object does not have to be a file in the sense of an excel spreadsheet or digital photos from your friend's wedding. An S3 object can just be a sequence of random data, which would be called a file if stored as ".dat" in a filesystem, but is not considered a file in the same abstract sense as a Word Document.

From a programming perspective files tend to come with a number of traits, like options for sequential access, random access, or appending. A file can often be modified without re-writing the entire thing. When you are programming with S3 objects, the only way to "append" something is to GET the object, retrieve the value from the object, append the data to that value, and then use PUT to upload entire object back into S3.

In this way, it is similar to FTP, except that FTP is most definitely about files and filesystems. S3 is not just for downloading files, but for retrieving any kind of data, usually directly into an application.

Given that S3's target audience is software developers, the programming version of the term "file" is more appropriate.