IRRE Thumbnails are not rendered for images with extension written in capital letters
Since a recent change thumbnails are not generated for images as inline records anymore if their extension name is written in capital letters (e.g. JPG instead of jpg).
The InlineRecordContainer does not correctly get the file extension.
The proper way should be via File->getExtension(), which would always give the lowercase version. At the moment it is fetched through the method File->getProperty('extension'), which gives the extension as it is (i.e. in capital letters).
This means that the following check if the file extension is an image file extension fails for the capitalised version.