Mechanism for chloromethylation of benzene with formaldehyde and HCl
This reaction is chloromethylation, similar to Blanc chloromethylation, but using $\ce{AlCl3}$ as co-catalyst instead of $\ce{ZnCl2}$. These are reactions belonging to a group related to Friedel-Crafts reactions, but characterized by usage of protonation instead of coordination with molecular Lewis acids

Mechanism of your reaction (not accounting for influence of co-catalyst) can be written as:


(source)

$\ce{AlCl3}$ coordinates to $\ce{HCl}$ creating adduct ($\ce{HAlCl4}$) which is much stronger acid.