Do not allow screenshot upload outside of the "Upload screenshot" area margins

[softvision-raul-bucata]

URL: https://webcompat.com/issues

Environment: Operating system: Windows 10 Firefox version: Nightly 107.0a1 (2022-10-04) (64-bit)

Steps to reproduce:

1. Go to https://webcompat.com/issues
2. Complete the form submission until the "Upload Screenshot" area is shown.
3. Drag and drop the screenshot file outside the "Upload Screenshot" area.
4. Observe the result.

Expected Behavior: The file can not be uploaded/a forbidden icon is shown.

Actual Behavior: The file is opened in a new tab, with file: URL.

Notes:

1. Screenrecording provided:

2. Reproducible on Chrome as well
3. Upload inside the designated area works as expeted.

Watchers: @softvision-oana-arbuzov @softvision-raul-bucata @ClintNase