Order of halide nucleophilicity in DMF
It depends on the ability of the solvent to solvate anions. If you draw structures of $\ce{DMF}$ with its resonance structure and check the side of the molecule that should interact with anions, you will see, that methyl groups prevent the interaction pretty much. Same for $\ce{DMSO}$. Thus these two solvents are commonly used to enhance the reactivity of nucleophiles. They solvate cations well but anions poorly. Maybe a bit strange, but it works well and it's pretty reliable.