Yes, it's normal for KF8 to have .mobi/.azw as the filename extension as they are really just specially-assembled files within the old Mobipocket-style wrapper. Amazon only gives filename distinctions on files downloaded via the hardware devices and via the USB transfer option on their website. Otherwise stuff coming through the K4PC/Macs all has the same extension regardless of what type it is.
I'm not sure there's an easy way to tell the difference, but you can use the kindlestrip.py script within
Mobiunpack utility (go to the latest post for the newest version) in the Mobipocket forums to see what parts exist by splitting them if they're there.
There's no way to direct-edit any Mobipocket/KF8 files and you'll have to unpack or convert and recompile (best to keep an ePub source and make changes there before converting to "final" mobi format).