Alder/Willow Fly for me. I'm no expert on Amazonian or Colombian distribution of the two, and I would question "expected" species in some cases here - shy of a calling bird or excellent evidence, "expected species" get reported as the "expected species" and you get a chain of circular logic that might not be based in anything terribly authoritative.
As far as the other suggestions:
Cnemotriccus (Fuscous Fly - cabanisi subsp I believe in Leticia, in any case not duidae) is a different bird. This bird lacks any hint of warm tones / rufous in the wing bars or secondary panel, breast is too clean, doesn't have a superciliary, no warm tones to the facial markings, bill is too short and thick and perhaps not enough of a hook, tail is too short and seems to lack any hint of a small notch.
Lathrotriccus (Euler's by range) is a better fit structurally. However this bird lacks almost any hint of the expected warmer tawny / olivaceous / brown tones above, doesn't have a terribly marked breast, doesn't seem to show any warmth to the outer retrices, has a dark mandible on a perhaps too short bill. I also feel like Euler's is more bull-headed looking than Empids generally are. It lacks any warmth or tawny tones in the wing bars, and the yellowish undertail covert wash strikes me as something I've never seen on Euler's anywhere in its range.
