Fix erroneous file_type handling in certain NCL diagnostics

[zklaus]

Description

Several diagnostic scripts try to guess the name of the plot result file. However, the way that they are determined is different and the guess is wrong. This PR changes this by using the actual filename as given by the wks.

---

Before you get started

• [x] ~☝ Create an issue to discuss what you are going to do~

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• [x] 🛠 This pull request has a descriptive title
• [x] 🛠 Code is written according to the code quality guidelines
• [x] 🛠 Documentation is available
• [x] 🛠 Tests run successfully
• [x] 🛠 The list of authors is up to date
• [x] 🛠 Any changed dependencies have been added or removed correctly
• [x] 🛠 All checks below this pull request were successful

New or updated recipe/diagnostic

• [x] 🧪 Recipe runs successfully
• [x] 🧪 Recipe is well documented
• [x] 🧪 Figure(s) and data look as expected from literature
• [x] 🛠 Provenance information has been added

---

To help with the number of pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository