Given the repeated "tt" and "rm" patterns, one common guess is Atbash (A↔Z, B↔Y, etc.) or a Caesar shift.
Given the impossibility of solving without more info, my best guess is the author used to obscure a phrase like "open the file..." or something similar, and "Download-" is plaintext indicating the action. Download- nwdz lshrmwtt khlyjyt fatht layf ttshrmt...
Maybe the cipher is ? nwdz reversed → zdwn — no. Given the repeated "tt" and "rm" patterns, one