Make image plugin to attempt to detect SVG types

[ssjhv]

If known image detection fails, and if the beginning of the data is either XML or SVG header, then set mimetype to image/svg+xml.

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[nfelt]

Thanks for the change to add SVG support!

Do you mind adapting this logic to integrate with imghdr? The module has a imghdr.tests variable that you can extend to add support for new image types. That way SVG doesn't have to be special-cased as a fallback here. See https://github.com/python/cpython/blob/3.6/Lib/imghdr.py#L32

I believe the code would look like

import imghdr

def detect_svg(data, f):
    if data.startswith(b'<?xml ') or data.startswith(b'<svg '):
        return 'svg'
imghdr.tests.append(detect_svg)

Then you'd just add the appropriate entry to _IMGHDR_TO_MIMETYPE and the code here should work without any changes.

[ssjhv]

Thanks! I'll try to see if making changes to imghdr is acceptable.

[googlebot]

CLAs look good, thanks!

[nfelt]

@ssjhv Just to be clear, I didn't mean actually changing the imghdr module itself. The module provides a mutable list of tests as the "extension API" where you "register" a new test by appending it to that mutable module-level list. So the code I'd sketched out would just be added to the top of the images plugin file.

[nfelt]

LGTM, thanks for the update.