Trying to display an image file that doesn't have an image extension but has the mime type set correctly to an image type results in the following error:

Unable to embed resource: attachment of type application/octet-stream

Confluence should respect the mime type and display the attachment as an image. This situation can arise relatively easily by using the chart macro's attachment support which allows the attachment name to be anything. See http://confluence.atlassian.com/display/~jnolen/Test. T