MIME type detection breaks on some files

[corbindavenport]

The uploaded MIME type for files isn't stored anywhere after upload, so if the MIME type can't be detected automatically by the mime library during the download request, the downloaded file can't be opened unless the user adds a file extension manually. This affects PICT screenshots uploaded through Internet Explorer on classic Mac OS, and possibly other use cases where a file extension is not provided.

ImageShare needs to store the MIME type as provided during upload alongside the file (probably in an array?), and then delete that when the image is deleted. Alternatively, a file extension from the MIME type could be added during the Multer upload function when the file is renamed, if an extension is not already present.