CouchDB Attachments Cannot be Opened
The problem seems to be the ContentType header that you have specified during the attachment creation.
The correct header name is Content-Type
The curl command should be:
curl -vX PUT 'http://localhost:5984/categories/Jan/example.pdf?rev=4-2c0b1c27daca6d2a3c375b0f879a8967' -d@example.pdf -H "Content-Type: application/pdf"