fileserver: set Content-Length for precompressed files

[WeidiDeng]

Fix 7250.

Assistance Disclosure

No AI was used.

[francislavoie]

So I guess #7042 wasn't enough?

Windows test failure is:

=== RUN   TestAdminHandlerBuiltinRouteErrors
    admin_test.go:282: setting up admin server: listen tcp 127.0.0.1:2019: bind: Only one usage of each socket address (protocol/network address/port) is normally permitted.
--- FAIL: TestAdminHandlerBuiltinRouteErrors (0.00s)

Not sure what the deal is, this seems new, it used to be fine?

[mholt]

@francislavoie I think the tests are just flaky. I see equivalent errors on my local Linux box quite often. I re-ran the tests and they pass now.

[mholt]

Thanks Weidi!