This issue was originally reported by @Rsteen at https://commons.wikimedia.org/wiki/Commons:Village_pump/Technical#False_warning_for_png_files.
Rsteen was attempting to upload File:Christian Andreas Schleisner - Børn, der søger ly ved en båd på stranden - 1854.png and other files using Special:Upload from Firefox 76.0.1 (64-bit) and Windows 10 Home Version 1903.
Steps to Reproduce:
- Download a zoomable image >10 MB using https://ophir.alwaysdata.net/dezoomify/dezoomify.html
- Upload the PNG file to Commons using Special:Upload
Error from Special:Upload: File extension ".png" does not match the detected MIME type of the file (unknown/unknown).
MIME type is image/png, no error is generated
Firefox's MIME type detector correctly reported that the image had a type of image/png. Rsteen then uploaded the file without issue using Special:UploadWizard. I was not immediately able to reproduce the issue using Firefox 76.0.1 (64-bit) on Arch Linux.
Please note that this was done on a slow internet connection. First there was an error message saying that the servers were busy, please try again. When the retry was done the error message about the faulty png-file appeared. So this error can probably only be replicated on a slow internet connection.