With IBM BPM 8.5.7, you may notice that Attach File controls using the 'Associate with Process Instance' option generate generic error messages. After attempting to upload a file, the control presents an "Internal Server Error" validation popup and the following message is recorded in the browser console log:
Failed to load resource: the server responded with a status of 500 (Internal Server Error)
Encountered Error: Internal Server Error
Starting in 8.5.7, IBM merged Business Process Definition (BPD) and Case modeling into the new "Process," which is only available via the Web Process Designer. The internal server error identified above stems from changes to how BPM permits document changes with these merged content management controls. The 'Associate with Process Instance' option of the Attach File control, therefore, requires explicit permission to manage local documents.
Note: Existing applications migrated to 8.5.7 will continue to work as before as they will still be defined with BPDs. However, the differences introduced by 8.5.7 Processes may catch long-time developers of BPM unawares when creating new Processes or using the "Convert to process" action on an existing BPD.
To fix this issue, navigate to Process Name > Folders > Folder Management and check the 'Allow locally managed documents' checkbox as shown here:
See more details on IBM BPM 8.5.7 Content Integration here