getExtension does not work as expected

[JWaters02]

Describe the bug

When testing #4255 I was the downloaded files are only being txt despite my PDS names matching the DS_EXTENSION_MAP, and with "override extension" being disabled.

I tested this both before the change and after the change and it seems to be a bug in both, i.e. in the getExtension function, so not to do with that change.

For example, MY.USERID.TESTJCL contains JCL members, but they are all downloading to .txt, despite the fact that "JCL" is in "TESTJCL".

When I disable the JCL support extension, the members in that TESTJCL PDS indeed do not open up as JCL. But obviously I've had it enabled the whole time, so I only noticed this issue once I had done the download on the PDS.

Note that I do have **/*.TESTJCL*{,/*} in my files.associations setting but since getExtension doesn't take files.associations setting into account, it doesn't have any affect for opening or downloading the files from Zowe Explorer (only for local files).

To Reproduce

1. Make PDS containing TESTJCL name
2. Make members in the PDS
3. Click "download all members" or "download member" and make sure "override file extension" is false
4. See that the members are downloaded as .txt instead of .jcl

Expected behavior

Since the PDS name contains "JCL" in it, the members should be downloaded as .jcl

Screenshots

Desktop (please complete the following information):

• OS:
• Zowe Explorer Version:
• (Optional) Zowe CLI Version:
• (Optional) Do you have secure credentials enabled?
• (Optional) Are you using a custom credential manager?

Additional context

[github-actions]

Thank you for creating a bug report.
We will investigate the bug and evaluate its impact on the product.
If you haven't already, please ensure you have provided steps to reproduce the bug and as much context as possible.

[traeok]

#3131 mentions the concern of Zowe Explorer applying extensions. Should we reopen this?