Add .svg extension to supported image file extensions

[tjone270]

I've noticed that SVG images are not in the hardcoded list of supported image extensions. Having tested SVGs on my BookStack instance in the WYSIWYG editor these behave normally, so think it's worthwhile having this as an out-of-the-box supported image.

[ssddanbrown]

Related to #1103

Thanks for offering this @tjone270.
This is a path I've thought about and tested before:

• SVG Image and Drawing Support #3452
• Draw.io using SVG to support links within diagram #1170 (comment)

My issue comes down to SVG being a complex format that we can't assure wider support for (like in PDF exports) and which brings a wider range of expectations upon normal images (interactivity).
Edit: Also need to think the security implications of displaying SVG images, especially when accessed direct via browser in public storage. Quite likely a range of security concerns.

Maybe the demand/value is enough for implementation regardless of compromise.
Think I need to dwell on this, I might ask about this via the issue for feedback.

[thajohns]

With regards to security, I think this chapter from the SVG standard is useful.

[morrolinux]

I deperately need this feature for migrating to bookstack.
How about making it an opt-in until all the rest is figured out @ssddanbrown ?