Currently, Fisheye relies on the VCS to tell what is the correct mime-type to be used for each file and each version of the file. If the mime-type defined in the file is not the correct one, special characters are show with a � (? in a diamond).

The latest version of the file can be easily modified to have the correct mime-type, however changing the older versions may not be trivial.

Fisheye could have an option to overwrite the old versions mime-types with a custom one.

The current workaround would be to change the each version of the file. The following tutorial shows how it could be done:
http://blog.philippheckel.com/2011/02/01/altering-old-svn-revisions-removing-confidental-data-from-subversion-repository/