Getting image size based on header

[aldm alert]

Hi,

is there any library or gem to get image size (in pixels) based on
image headers. Image headers should contains bytes that describe image
size, which is different for different formats - jpg, png, gif
etc.

This size does not need to be same as actual size of image and this
vulnerabily can be used for attacks:

https://hackerone.com/reports/390

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