Learn Hardcore Spanish (Spain): Me dejé el paraguas en la cafetería.

Here me is part of the very common Spanish pattern dejarse + object, which often means to leave something behind / forget something somewhere.

So me dejé el paraguas does not mean I left myself the umbrella. It means I left my umbrella behind.

The me matches the subject I:

me dejé = I left behind
te dejaste = you left behind
se dejó = he/she left behind

Is dejarse really reflexive here?

It is pronominal in form, but not truly reflexive in the literal English sense.

A literal reflexive idea would be something like I washed myself. But in me dejé el paraguas, the meaning is idiomatic: I accidentally left the umbrella behind.

So it is best to learn dejarse algo as a useful pattern:

Me dejé las llaves en casa.
Se dejó el móvil en el coche.

What is the difference between Me dejé el paraguas en la cafetería and Dejé el paraguas en la cafetería?