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 davidg@nikhef.nl.