Prelude> :type sqrt
sqrt :: (Floating a) => a -> a

... note that the return type is the same as the argument type. From a
commonsense point of view, one can see part of the reason for the
complaint about Int as being that for things like sqrt (2::Int) you
couldn't return an Int answer (at least a correct one!).

Of course,

Prelude> :type mod
mod :: (Integral a) => a -> a -> a

mod expects an integral n, and you already declared n to be Int.

How about,

proper_divisors n = [d | d <- [1 .. (floor . sqrt . fromIntegral) n], mod n d == 0]

By the way, though I actually much prefer your convention, upper and
lower camel case seem to have established themselves as the Haskell way,
so normally one would name your function properDivisors.