I too am a happy user of Fastmail, but I am plenty dissapointed with the file storage, which was one of the main reasons I switched to them 2 years ago (and not to a competitor). Their support in both cases was just awful and more or less ended in a "sucks to be you message":

1. I wanted to sync a folder from my Windows laptop to the storage through Webdav, using the Windows Explorer (which they list as a supported client). I would get speeds of maximum 5kb. Opened a ticket, went back and forth and the customer support person suggested that I try with another (paid) client to see if the speeds are better. I downloaded a trial, tried it out and it was indeed better. Reported back and the person says something summarized as: see? here you go, just use that client (that cost another 50 euros)

2. I find this one much worse: they disabled support for FTP access to the storage. I get an email 1 (ONE) day before saying something like: hey, there's no FTP starting tomorrow, we did send an e-mail but didn't include _you_, here's 15 dollars for your trouble. I was using FTP actively to back-up the website of my small business nightly. Of course I email support and tell them hey, c'mon, this is unnaceptable, you can't just tell me one day before, give me a solution, something. The reply came almost immediately and said: we sent an e-mail informing everybody 3 months ago, deal with it (???).