Add Markdown (*.md) to the "custom files" file selector in the Character File Upload

When uploading a file in Character Context or Character memory, the Custom Files types lists many files (.pdf, .txt, .html, etc). It does not have the .md file type which is now among the most common text file.

I’m aware there is a workaround by selecting “All files", but I think .md files should be considered a text file by default.