When an image is uploaded to the DAM, its MIME type is saved as the property jcr:mimeType on the binary node. Similarly the file name is saved as @fileName and the extension as @extension.

However, when rendering a resized image, getOutputformat() of the class info.magnolia.dam.imaging.responsive.ui.imaging.ResponsiveImageOperationChain parses out the extension of the asset node's name which is assumed to be the name of the originally uploaded asset.

This is very brittle and means that when the node name is something other than a filename with an extension the resizing process fails, resulting in the error:

java.lang.IllegalStateException: untitled has no known entry in MIMEMapping
info.magnolia.imaging.AbstractImageStreamer.getMediaType(AbstractImageStreamer.java:51)
info.magnolia.imaging.DefaultImageStreamer.serveImage(DefaultImageStreamer.java:64)