The file name of images in HTML reports have no extension, this is somehow not typical if comparing to the general practice in Web. This behavour can yield problems in some server setups like reverse proxy. We have this situation using an apache reverse proxy in front of our server delivering HTML reports. The proxy sets the content type for the image files depending on the file extension. If no one is found, the default content type is set, for example text/plain, a configured value in the proxy.
Delivering images with a wrong content type is an issue for some browsers like IE and Firefox. The Firefox behaves randomly in this case, sometimes showing the images, sometimes not.
Note that setting the default content type in the proxy configuration to a proper value is not a solution for us, because we have different image formats in our reports and this would not only affect out reports, but potentially also other application areas.
Our feature request aims to create an option in Jasper Reports for forcing a report generation that preserves the image file name extensions.