A file not found error shows when loading the display form on an item that has attachments with special characters.
The error shows when editing or viewing the display form where the attachment control resolves to show all attachments for the item.
This is a known issue and will be fixed in a future release. As a precaution please don't use special characters in file names.
Please use the following workaround:
1) Switch to use the OOTB SharePoint forms and delete the attachment with the special character in it.
2) Use the Attachment SmartObject > Remove All Attachments Method to delete the Attachment. (The Remove Attachment Method doesn’t work against special characters).
3) Build validation in you form to validate the file name for special characters and based on that block upload.