When the file upload functionality is not working in your system then deploy them as a project.
To upload multiple files from one account to another account, create a VS code project and add the files that need to be deployed. (The path must be in the correct format)
Then connect the target account to the VS code.
Validate the project against the new account.
If there are no errors, deploy the project to the connected account.
By using this way we can upload multiple files into the target account.