<vaadin-upload> accept regex failure when mimetype contains '+'

[srijken]

Description

I added an uploader for SVG files like this:

<vaadin-upload accept="image/svg+xml"

Because the regex is calculated incorrectly, it doesn't accept svg files.

Regex is calculated to be ^(image/svg+xml)$, but the + sign should be escaped here.

Expected outcome

I can upload SVG files

Minimal reproducible example

<vaadin-upload accept="image/svg+xml"></vaadin-upload>

Steps to reproduce

1. Add the snippet above
2. Upload svg file
3. File is rejected

Environment

Vaadin version(s): 23.1.4
OS: MacOS 12.3.1 / Chrome 104.0.5112.101

Browsers

Issue is not browser related