I just imported a broken SVG, edited it and exported it via Adobe Photoshop. However, Photoshop then embeds a PNG in an href Base64-encoded...
Neither the SVG-Checker nor the Commons Upload assistant were able to tell me what's the real problem. This ended up in discussions and confusion on the dewp "Grafikwerkstatt".
I suggest to implement a check for this case:
If the following text is found in the file
-> tell the user that this is a PNG embedded into a SVG file.