We are on AEM 6.3 sp2, with the corresponding Forms add-on package (OSGi).
We also have a form with the attachments/file upload component. With the configuration we had, any forms that had an attachment uploaded would NOT submit. After months of an open Daycare ticket, and rebuilding the publish environment, we determined that there was a debug flag turned on. It's the "Adobe Granite HTML Library Manager" setting, which after we set the debug flag as unchecked, uploaded attachments started working. I'm sharing the info in case anyone else may come across this issue.
Although I still believe that the debug flag shouldn't prevent a form submission with attachments from completing...but it did.