Understanding File Upload vulnerabilities with MIME checking - Overthewire.org - Natas 13

The application has more defenses now, specifically it checks and makes sure that the upload file is an image. However, can this check be bypassed, or set to validate a backdoor as an image?

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[TheTruthfly]

thank you very mutch ! i understand now ;)

[j.stan]

Thanks! If you are having trouble uploading an image you can delete the gibberish at the beginning and add something like GIF234941339390 for example and below the code. I couldn't upload mine until I did that even though I didn't everything correctly (in my eyes at least).

[johndoe-lxzh]

Repeater keeps telling me that "file is not an image", though moments ago i successfully uploaded file with the same raw request.