Allow viewing of plaintext MIME type in Virtual Repositories

Current functionality lets someone view plaintext content in the Web UI if it's defined in mimetypes.xml but ONLY if it's viewed via the local repository URL.  Please make this work in virtual repos as well since multi-site HA topology relies on virtual repos to present a unified endpoint/interface.  Of note: XML type files are viewable in both local and virtual repos just fine.  With that already in place, I would expect that this shouldn't be complicated functionality to add.