convert: Handle mmproj model output filename properly

[Galunid]

Currently when output filename is not specified, script generates file called mmproj-model-dir without any extension, in a parent directory.

This PR changes the way default filename is handled. With this change generated file will look like this model-dir/model-dir-ftype-mmproj.gguf.

Note this uses -mmproj suffix, instead of mmproj- prefix. I opted to follow the established convention.

Example:
Current: Qwen3VL-2B-Instruct/Qwen3VL-2b-Instruct-F16-mmproj.gguf
Previous: mmproj-Qwen3VL-2B-Instruct, note unspecified filetype, wrong directory and missing extension

[ngxson]

Note this uses -mmproj suffix, instead of mmproj- prefix. I opted to follow the established convention.

I would keep the mmproj- prefix as it prevent users from downloading the wrong file, especially when long file names can be truncated when display on web / on local file browser

[Galunid]

Sure, that makes sense. I updated this in my latest commit.