Windows 11 Weird "Save As" Filetype Issue

Brenski — that “Save as type: Astronomical Image” text is coming from Windows’ file-type registration for the .JPG/.JPEG extensions, not from the website. In other words, something on the PC has (accidentally or intentionally) changed what Windows calls a .jpeg file, so the common file dialog is now showing that label.
The good news: it’s usually just a file association / ProgID mix-up and is easy to reset.

Fix (recommended): Reset the default app for .JPG / .JPEG​

• Settings → Apps → Default apps
• In the search box, type .jpg
• Click the current default, set it to Photos (or your preferred viewer)
• Do the same for:
• .jpeg
• (optionally) .png, .webp, .jfif
• Close all browser windows and try “Save image as…” again.

Also in Settings → Apps → Default apps, there’s a Reset button (“Reset all default apps”) — that can fix it quickly if multiple image extensions got changed at once.

Quick confirmation (to prove it’s the Windows association)​

Open File Explorer and find any .jpeg you already have on disk, then enable the Type column (Details view). If it also says Astronomical Image, that confirms it’s the system-wide .jpeg file type description.

If it immediately “changes back” again​

That usually means a program is re-registering the extension (often image software, camera/RAW tools, or specialty viewers). If you tell me:

• which browser you’re using (Edge/Chrome/Firefox),
• and whether you installed/updated any image/astro/camera software recently,

…I can help you identify the likely culprit and stop it from taking over the associations.
If you want, I can also give you a couple of safe command-line checks (assoc / ftype) to show exactly what .jpeg is currently mapped to on your system.