That doesn't mean it's not a pcap file - it means the packets in it
don't have 802.11 headers.

A lot of 802.11 adapters can be configured to provide to the host fake
Ethernet packets, rather than 802.11 packets, and a lot of 802.11
drivers will, by default, configure the adapters to do so.

On Windows prior to Windows Vista, that's how *all* the drivers work, as
far as I know; the networking stack doesn't handle 802.11 headers. In
Windows Vista, the networking stack can handle 802.11 headers, but not
all drivers have been changed to work with the "Native 802.11" mechanism
- and, even for those that have, WinPcap doesn't put the adapter into
monitor mode, so they won't supply 802.11 headers.

So if you've captured on Windows with a WinPcap-based application, such
as WinDump or Wireshark, you won't have an 802.11 capture.