Error uploading: "API upload contained multiple file parts"

I use Claris FileMaker to upload a file via https://upload.box.com/api/2.0/files/content

I use following cURL: 

"--location  --header \"Content-Type: multipart/form-data\" --header \"Authorization: Bearer XXX\" --form \"attributes=@$JSON\" --form \"file=@$Fil\" -D $header"

I get HTTP/1.1 400 Bad Request with this in return: 

{"code":"bad_request","help_url":"http://developers.box.com/docs/#errors","status":400,"message":”API upload contained multiple file parts","type":"error"}

I dont see any info on “API upload contained multiple file parts” in the link, or on the web. Someone knows what it mean and how to solve it?