Download- Fydyw Tjss Ly Lhm Mharm Mn Tht Qb A... -

Wait — "Download-" is plaintext. Then "fydyw" — maybe it's "email"? No.

Given this is a puzzle, and you asked for a "write-up", I'd conclude the intended decoding is a (or 21 forward), yielding: Download- fydyw tjss ly lhm mharm mn tht qb a...

Shift backward by 1 didn't work. Maybe shift forward? Wait — "Download-" is plaintext

Let's brute force Caesar mentally: