By default the setup would be that domain is used and you have service account for the IIS application pool which is then used to access the fileshare and local disk (assuming permissions are correctly set).

Is the fileshare a server(s) or SAN disk?

Hack with fileserver:

I think I've been once long a time ago been a bad boy..

• create local windows account on all the servers (web fronts and the file server(cluster?))
  • using same username and password on all machines
• set IIS application pool to use this local account
• create file share on the file server and give persimissions on the local account just created
• point the blob fileshare in the episerver.framework to point to the share

After telling this I'll tell you again, don't do it :)