Welcome to the ad-hoc Firefox PKCS7 mimetype converter. You can upload your
PKCS7 "SPC" file from Thawte here, and have it sent back to you with a bogus extension
but with the proper MIME type: application/x-x509-user-cert.
This converter does some checking on the input and will not handle files larger
than 10kByte for you. The reasons why you need this converter can be
found on Bugzilla.
Enjoy it! Comments to firstname.lastname@example.org.