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a couple of images I've downloaded today on my phone to share elsewhere and had to grab a file explorer to rename them
Which images in particular?
Possibly the extension doesn't match the mime type, try the attachment at https://www.lfgss.com/comments/13684952/ which has been deliberately misnamed as .png even though it's really a .jpg
gbj_tester-
The correct mimetype should be sent regardless of the file extension.
Jpgs are also auto converted to webp for clients that support webp, and yet this is done without a redirect so the file extension remains intact.
We also don't trust that something that calls itself a jpg is an image. So regardless of file extension, we want to make sure an exe isn't uploaded containing malware, so every image is inspected, and during that we may adjust the mimetype.
Velocio
Is the forum affixing -large and maybe other stuff to images now that you're allowing bigger ones? There's been a couple of images I've downloaded today on my phone to share elsewhere and had to grab a file explorer to rename them so they're recognised.