fix(mm): only add suffix to model paths when path is file

[psychedelicious]

Summary

I noticed that diffusers/folder models with a . in the model source had everything after the . appended to their destination path.

Example model:

depth_sdxl = StarterModel(
    name="Depth Map",
    base=BaseModelType.StableDiffusionXL,
    source="diffusers/controlNet-depth-sdxl-1.0",
    description="Uses depth information in the image to control the depth in the generation.",
    type=ModelType.ControlNet,
    previous_names=["depth-sdxl"],
)

This model's source ends with .0. It was installed into sdxl/controlnet/Depth Map.0, when it should be installed to sdxl/controlnet/Depth Map.

I added an extra check to only add the suffix to the destination path when the model is a file. Single-file models still get the correct file extension and folders use the preferred name.

Related Issues / Discussions

This is a follow-on to #8352. It's been an issue for a long time.

QA Instructions

I used the SDXL Depth Map and CyberRealistic models as tests for this. They isntall to the correct locations.

Merge Plan

n/a

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)