Add a parameter to FileDialogManager.SaveFile to append an extension.

[whatston3]

Title.

Adds an optional parameter called appendExtension to FileDialogManager.SaveFile, defaulting to false (preserving existing behaviour - shouldn't break anything as-is). When set to true, if a file is selected for saving, the extension is added to the file if the entered file name:

1. Doesn't exist,
2. Doesn't have an extension already,
3. Wasn't selected with a wildcard filter.

IFileDialogManagerImplementation.OpenFile, SaveFile edited to receive the filter index to get the expected file extension to append to the file.

To test:

1. Checkout 2026-09-07-append-extension off of my ss14 repo
2. Build and run (might need to add me as an RT remote to get the submodule, but it's the head of this PR.
3. Run golobby, enter the character menu.
4. Export a character to "bababooey", you should write to "bababooey.yml".
5. Export a character to "plonk.yml", you should write to "plonk.yml".
6. Export a character to "funny.txt", you should write to "funny.txt".
7. Rename "funny.txt" to "funny" and save to "funny" - you should rewrite over "funny", and not create a file called "funny.yml".

[metalgearsloth]

Am I high or is this possible to sandbox break because filedialogfilters are entirely user input and we're manually constructing the path here?

[whatston3]

The extension groups shouldn't be user input - in the existing test cases, that should be "yml" and "csv".
I mean, they could be, but why would you do that?

[If it is a problem, and if it can be fixed, I'm all ears.]

[metalgearsloth]

I don't think we have any existing restrictions around file formats, e.g. if some fork wants to use their own format, or wants their own tigher whitelist, which I don't know if we want to support or not. it may become a problem when video resources get implemented so maybe better to just allow any extensions to be used for filters.

[Mackamuir]

Epic