fix(frontend): see knowledge in custom brains (#2278)

# Description

Please include a summary of the changes and the related issue. Please
also include relevant motivation and context.

## Checklist before requesting a review

Please delete options that are not relevant.

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented hard-to-understand areas
- [ ] I have ideally added tests that prove my fix is effective or that
my feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged

## Screenshots (if appropriate):

frontend/app/studio/[brainId]/BrainManagementTabs/BrainManagementTabs.tsx

@@ -39,7 +39,7 @@ export const BrainManagementTabs = (): JSX.Element => {
       isSelected: selectedTab === "Knowledge",
       onClick: () => setSelectedTab("Knowledge"),
       iconName: "file",
-      disabled: !hasEditRights || brain?.brain_type !== "doc",
+      disabled: !hasEditRights || !brain?.max_files,
     },
   ];
 

frontend/lib/context/BrainProvider/types.ts

@@ -33,6 +33,7 @@ export type Brain = {
   brain_type?: BrainType;
   brain_definition?: ApiBrainDefinition;
   integration_description?: IntegrationDescription;
+  max_files?: number;
 };
 
 export type MinimalBrainForUser = {