I'm using a sandisk 32GB SDHC which has worked fine even though I hadn't formatted it - just used from the last phone I had with all the same files (mainly music/some photos).
As discussed on other posts, it has been reported to work, although you're probably going to have to format it to fat32 first -- exfat is not supported.
QWERTY fanboy: Nokia N900 (Maemo 5), Sony-Ericsson Xperia Pro (Android 4.0 ICS), Samsung Captivate Glide (Android 4.2 JB) Still miss the awesomeness of the N900...
If you try to format your SDXC 64 GB card in windows, it will fail because it is crippeld. Either use Linux to format it. I formated mine (using Ubuntu) for FAT. This did the trick for me. The Gallery and Documents apps (Jolla apps) does see files I copied to the 64GB SDXC card after I inserted in my Jolla.
I have a Sandisk 64GB micro SDXC in mine. As standard the card is formatted to the exFAT standard which Jolla can't read. I reformatted mine via an SDXC compatible USB card reader plugged into my Windows XP laptop. I used a small utility that can format any size drive to a single FAT32 partition
I have a SanDisk Ultra 64GB SDXC card. At the moment it is formatted as btrfs. With the original release version on sailfish, Jolla automatically mounted the card and I was able to play music from it. With the latest release, it does not, so I need to mount the card manually from the terminal. Yeah, I know, I could format it to FAT, but I could also tweak my Jolla so that it will mount the btrfs -formatted card automatically again. I have not decided yet which way to go. I might test other filesystems first (like EXT4).