It's not just you.
I could not load it either. macOS Sierra 10.12.3 and FLUX Studio 0.7.1
1) FLUX Studio has been having issues with large files, and that one's 82Mb
2) (I think this is the real problem) I suspect that file was saved and uploaded as an ASCII STL file and not binary.
Try this one instead.
Ergonomic Magic Mouse
I just loaded it up in S3D and exported again as binary, now it is 15Mb and loads just fine in FLUX Studio.
FYI, to print clean and fit well without a lot of post-printing work, you will HAVE to print that with supports, and you will want to get them just right. If you are using only FLUX Studio, I suggest Cura and adjust settings and use Preview and zoom in/out and readjust until you get it looking good, too much support is going to make a cleanup nightmare, too little and it's not going to print well.
If you have the option, Simplify3D will handle it easily. But if not, just take your time and adjust Cura settings and it will work that way too.