One alternative would be to encode the PKCS#1 format RSA public key as a PEM file, and then use the MIME type:
"application/x-pem-file"
References:
- http://www.cryptosys.net/pki/rsakeyformats.html
- http://pki-tutorial.readthedocs.org/en/latest/mime.html
Note: "application/x-pem-file" is not registered ... obviously! ... but is referenced in a number of catalogues of "file types".
You probably ought to deliver the key over HTTPS ...