Web Clipper 'Create Bookmark' option creates .txt file in Firefox

[uggrock]

Original Issue: tagspaces/tagspaces#1637

Describe the bug
When I try to download a .url file with the 'Create Bookmark' option, the extension .txt gets added to it automatically, which makes it more annoying to use.

To Reproduce
Steps to reproduce the behavior:

1. Go to TagSpaces extension button
2. Click on 'Create Bookmark'
3. Click on 'Save'
4. Observe downloaded file
5. See error

Expected behavior
For the downloaded file to have a .url extension.

Screenshots

Desktop Application:

• Operating System: Windows 10
• Browser - Firefox (also tried Chrome, problem does not occur)
• TagSpaces Web Clipper Version 3.1.9

Additional context
I'm pretty confident that this is a problem on the Firefox end and expect there's nothing you can do; but I thought it was worth making you aware of it, and that I might be wrong.