A bit late but it seem the question was misunderstood. He just only had the base64 content of the image, not the full data URI.
I wrote here for anyone who encounters with this quest, you can read the first character of content content.charAt(0). By base64 image content if the first char is:
“https://stackoverflow.com/” : jpg
‘i’ : png
‘R’ : gif
‘U’ : webp
So for your case, it is ‘jpg’.