Fix code scanning alert no. 21: Uncontrolled data used in path expression

[amgoogle]

Fixes https://github.com/google/litmus/security/code-scanning/21

To fix the problem, we need to ensure that the filename parameter is sanitized and validated before it is used to construct the temp_filename. We can use the werkzeug.utils.secure_filename function to sanitize the filename, ensuring it does not contain any special characters or path traversal sequences. Additionally, we should normalize the path to ensure it stays within the intended directory.

1. Import the secure_filename function from werkzeug.utils.
2. Use secure_filename to sanitize the filename parameter.
3. Construct the temp_filename using the sanitized filename.

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