Add correct mimetype for .mjs files

[huchenlei]

Description

We have

stable-diffusion-webui/modules/ui_gradio_extensions.py

Lines 22 to 23 in 1c0a0c4

	for script in scripts.list_scripts("javascript", ".mjs"):
	head += f'<script type="module" src="{webpath(script.path)}"></script>\n'

loading extension's javascript file as js module. However, without this PR, mjs is not sending out with correct mime type by FastAPI.

Blocking Mikubill/sd-webui-controlnet#2822.

Screenshots/videos:

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests