Created attachment 132425 [details]
Writer document with two inserted images for testing Save image

When an image is inserted in a document, right clicking on the image and choosing Save (or Image > Save in Impress) allows to select the Image type (although the original type is suggested).

E.g. When the user saves an inserted JPG image as PNG, it is saved as imagename.png but it is in fact a JPG image with a wrong extension.
This can be easily verified by opening imagename.png with a text editor (or an image viewer that warns about such errors such as IrfanView in Windows)

This error affects at least Writer, Calc Impress and Draw (did not test Base; Math does not allow to insert images)

This is a regression inserted somewhere between version 3.6.7 and 4.0.6
Tested under Windows 10 Home x64.