As far as I know, utilitarians consider that only the consequences should be considered as the calculation of the morality of an action.

Since a mistake and a lie differ only in their intent, are they equivalent according to this philosophy?

The evaluation of such a thing looks different under different utilitarian approaches.

Act utilitarianism

Act utilitarianism is a utilitarian theory of ethics which states that a person's act is morally right if and only if it produces the best possible results in that specific situation.

Source: Wikipedia

So the first thing that's important to understand is that under this approach an act utilitarian will consider certain lies - those that result in more happiness than not saying the lie - morally a good thing.

An example of this would be:

Thomas has stolen a thousand dollar from his millionaire friend. His friend asks "You are my friend, I trust you 100%, did you steal that money?". Thomas - an act utilitarian - confidently answers he did not, as telling the truth would make both him and his friend unhappy.

This brings up back to your original question where a lie can be morally good or bad in the same way a mistake can be.

Rule utilitarianism

Rule utilitarianism is a form of utilitarianism that says an action is right as it conforms to a rule that leads to the greatest good, or that "the rightness or wrongness of a particular action is a function of the correctness of the rule of which it is an instance".

Source: Wikipedia

Under this philosophical approach the main question is about how the rules are defined. It's easily conceivable that a rule utilitarian would thus take the approach that "on average" the long term effect of lies makes people unhappy, and thus lies - as a rule - are morally wrong.

Obviously in that case there is a strong distinction between a lie and a mistake as rules are considered an abstraction which ...

Rule utilitarians argue that following rules that tend to lead to the greatest good will have better consequences overall than allowing exceptions to be made in individual instances, even if better consequences can be demonstrated in those instances

Source: Wikipedia

And beyond those two there are of course countless of other variants of utilitarianism, so read up on those as well.

Since utilitarianism is meant for people who are not all-knowing, only the foreseeable consequences count. And a mistake and a lie do not differ only in intent, they also differ in what the person knows, and, therefore, can foresee.

Depends on what you mean by "moral equivalence". If you mean that the consequences are equivalent, then yes, they are the same, but this is independent of utilitarianism. If you mean "both are bad", then they are not equivalent -- to an actor with imperfect information, what in hindsight is shown to be a mistake may have been a perfectly rational, ethical decision when it was made with the information then available to the actor.