Can't upload assets in Craft 3 CMS When I upload an image to assets, it just shows the progress bar loading, but it won’t upload my image into assets. Originally, this had worked before, but for some reason it suddenly just stopped working. I’m using Media Temple for my production. I checked my error log and it says: 


  Got error 'PHP message: PHP Warning:  File upload error - unable to
  create a temporary file in Unknown on line 0


I’ve checked my files permissions and changed /tmp to 777 and I’ve also changed my assets folder to 777 too. I even checked my disk space for the /tmp folder and it still have 25GB left so it’s not out of disk space.
I know this is a while ago but I ran in to the same issue and thought I'd share the culprit in my particular case.

I adjusted the upload_max_filesize value via a php.ini file from the default 2MB to 10MB, but I misspelled the configuration as upload_max_filesize = 10MB instead of upload_max_filesize = 10M. I was seeing the exact same error as the original poster. Once I fixed the setting, changing 10MB to 10M, the error went away.