Whether the data was retrieved over a network has nothing to do with it.

There are complementary ways of guessing what data you are working with -- guess based on the filename extension or sniff based on the contents of the file (or downloaded data).

There are a zillion reasons why the filename could be a lie -- e.g. a user could pick the wrong extension, or rename a file, or a tool could save a file using the wrong extension or no extension at all. Then again sometimes the contents of the file might not be enough, e.g.
```
foo() // bar
```
is both valid Python and valid JavaScript. :-)