If your user gets an error when trying to upload a File in their Feed:
A user must have Read and Edit access to the Contact object as well as the System Permission “API Enabled" in order to upload files.
- Profile assigned to the user to confirm the right level of access.
- Check the Organization-Wide Default Sharing Settings for the Account and Contact objects to ensure that these objects are not set to a Private or Public Read-Only model. Those restrictions will prevent the community user from having edit access to their related Contact record.