There are two possible causes for this error:
1. The file extensions is not allowed in xFlow.
2. The extension does not match the content type of the file.
For cause 1:
Add the extension to the allowed list:
1. Log into Service Desk as an Administrator and go to > Administration tab >Flow Analyst Interface > General
2. Open attachments.allowed-content-types > Edit
3. Add the extension and MIME type.
Example: To add .rar files:
Where .rar is the extension and application/x-rar-compressed is the MIME Type.
List of MIME Types can be found in the link bellow.
For cause 2:
Change the extension to the right one, so it does not fail when verifying the extension.