I think it would be more readable if _SendfileProtocol exposed a drain coro-method:

await proto.drain()
...
transp.write(..)

Where _SendfileProtocol's drain would look like:

async def drain():
    if self._can_write is None:
        return
    await self._can_write
    self._can_write = None

No.
The _paused attribute should be removed on writer resuming.
Doing it on draining is awkward and error prone.

Also I don't line drain name, it was always confusing.
We do access to attributes of other classes already in asyncio often enough.
A new private method for just replacing plain attribute access? Do we really need it?

A new private method for just replacing plain attribute access? Do we really need it?

Yes, because the code is super hard to read right now. drain isn't really confusing in this context.

Since it's a private method and the code will certainly look nicer/tighter, then why not?