DefaultImageGenerator and ImagingServlet (line ~98) used to hard-code the mime-type based on the imaging variation "outputFormat", which in many situations equals the filename extension:

// we need to set mimetype here explicitly because extension (and type) is determined at runtime depending on what the image op chain does
response.setContentType("image/" + outputFormat);

This was not correct, in particular for the extension "jpg" (a valid file extension for JPEGs) the resulting mime-type image/jpg is not the correct mime-type for JPEGs* (which needs to be image/jpeg).
While most browsers seem to cope with image/jpg, some (eg IE11) do not. The result is that image variations of jpg images are not displayed for us in IE11.

Fix of MGNLIMG-171

This problem was targeted with issue MGNLIMG-171 and now the ImagingServlet will correctly return the mime-type image/jpeg for extension "jpg" according to the given output format.

Remaining todo

However, CachingImageStreamer still persists cached images with the wrong property jcr:mimeType.

This is not causing the output to be returned with the wrong mime-type as the servlet will still correctly determine the content-type from the output format, but this should be fixed nevertheless.

See CachingImageStreamer line 220:

imageData.setAttribute(FileProperties.PROPERTY_CONTENTTYPE, "image/" + generator.getOutputFormat(parameterProvider).getFormatName());