MIME types are relatively flexible when it comes to creating new ones, in the sense that you can specify that a file is of type application/x-my-file-type and that s perfectly valid. As such, you should be able to specify, either through configuration files or scripts, that such MIME-type data will be sent with the file, but only on your host. Getting other sites to recognize your format will depend on each individual webmaster s (or hosting company s) decision.
Of course, that may not even matter to you. So, in short, YES you can define your own MIME-type (as long as you follow the rules), and YES you can configure your server to throw that MIME-type around. However, getting the rest of the world in compliance is the sticky-tricky part.
EDIT: To answer again in a different way: No, it is not difficult, you should be able to either do it yourself or have basic support walk you through it, depending on your implementation. It shouldn t cost you any money unless your host sucks, although this is my opinion and not necessarily a fact.
EDIT 2: To address your comment to this question (since the comment box doesn t have enough room):
If you are using Apache, you can create (or modify) an .htaccess file, which is very easy. For this example, say your file type is application/x-foo-file and the files are named with a .foo extension, you would need to add this line:
AddType application/x-foo-file foo.
If you aren t using Apache, you will need to look up your specific steps in whatever server you are using, although it is possible to use a scripting language (PHP, ASP, etc) to serve your files with the correct MIME headers.
PHP: header( Content-Type: application/x-foo-file );
ASP: Response.ContentType="application/x-foo-file"
