Signals notebook: Resolving "Invalid Notebook File" Errors When Uploading Previously Downloaded Files

Users may encounter an "invalid notebook file" error when attempting to upload previously downloaded .notebook files to Signals Notebook, even if these files were successfully uploaded in the past.

Issue Details:
- Previously downloaded .notebook files are rejected during upload
- Error message displayed: "invalid notebook file"
- Issue occurs with both previously uploaded files and new files
- Files have the correct .snb.notebook extension

Solution:
1. Ensure the user has valid access to both the source Signals Notebook instance (where files were originally downloaded) and the destination instance (where files are being uploaded)

2. Have the user re-download the notebook files from the source system:
  - Log into the source Signals Notebook instance
  - Navigate to the desired notebooks
  - Download the notebooks again using the export function

3. Upload the newly downloaded files to the destination system:
  - Without making any modifications to the file names or content
  - Use the standard import function in the destination Signals Notebook instance

4. If the issue persists:
  - Check browser console logs during the upload attempt (with "preserve log" option enabled)
  - Verify file extensions are correct (.snb.notebook)
  - Ensure the user has appropriate permissions in the destination system

Root Cause:
This issue is typically related to local file corruption or incomplete downloads rather than a platform problem. Re-downloading the files from the source system often resolves the issue.

Additional Notes:
- During system migrations or version updates, it's recommended to re-download notebooks rather than using previously stored local copies
- The "Export to File" feature availability may vary depending on the Signals Notebook version
- File compatibility between different versions of Signals Notebook may occasionally require re-downloading from the source system