Allow video format in the screenshot section

[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 an MP4 file in the "Upload Screenshot" area.
4. Observe the result.

Expected Behavior:
The MP4 file is uploaded.

Actual Behavior:
An error message is received.

Notes:

1. Screenshot provided:

2. Reproducible on Chrome as well
3. Reproducible for other video format files.

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

[rugk]

Same, wanted to do this here but could not do so: webcompat/web-bugs#126222 AFter all you can afterwards upload them to GitHub but well...