Experience Cloud: Error 'We couldn't upload the selected file' When Using Case Comments Publisher Component

To resolve this error, first determine whether the issue affects internal users as well. This helps narrow down whether the block is org-wide (such as file storage limits) or specific to Experience Cloud user permissions.

Step 1: Test File Upload as an Internal User

If internal users are also seeing this error in the Experience Cloud Site, try to have the user upload a file to the same case record internally from within the org to see if a more detailed error message is present.
For example, if you are testing this functionality in a Developer org or sandbox, it is possible the environment's File Storage has been exceeded and is currently blocking file uploads. Attempting to attach a file to the same case internally may provide a more detailed error: "Your organization is using all its file storage, so you can't add new files. Contact your administrator and ask them to purchase more storage."
If this issue is only occurring for external or Experience Cloud Site users, it is not possible to try uploading a file to the case internally for a more detailed error message.

Step 2: Check Community File Settings and Org-Level Upload Security

Administrators should review the setup for Limit Files in Your Community and the org's Configure File Upload and Download Security Settings to determine whether these settings are blocking file or attachment uploads for Experience Cloud users.

Step 3: Verify Edit Access on the Case Record

The only standard permission requirement to attach a file using the Case Comments Publisher is for the user to have Edit access to the corresponding case record. Verify the following:

• Object-level CRUD permissions: Confirm the affected user's profile includes Edit permission on the Case object. See Edit Object Permissions in Profiles for more details.
• Record-level sharing: If an administrator has set Organization-Wide Sharing Defaults for the Case object to "Private" or "Public Read Only," confirm the affected user has record-level Edit access to the associated case. If the user cannot edit the case record, an Admin needs to Set Up Sharing Sets to allow Read/Write access.

Step 4: Enable Debug Logging

If the Experience Cloud Site user has appropriate Edit access to the case object and record, set up Debug Logging for the Experience Cloud Site user, replicate the issue, then review the resulting logs to determine if customizations such as content triggers are blocking the file upload process.
Note: The Case Comments Publisher can only upload files as Attachment records and not as Salesforce Files — this applies regardless of whether the "Files are uploaded as Salesforce Files and not as Attachments" setting is enabled.