When opening a JPEG file that is intentionally misnamed as a .bmp, as of Paint.net 4.2, it gives this error:
"Application version: paint.net 4.2
PaintDotNet.Imaging.UnknownImageFormatException (0x88982F07): D:\src\pdn\src\SystemLayer.Native\Imaging\WICImagingFactory.cpp (892) : hr = spDecoder->Initialize( spStream, static_cast<WICDecodeOptions>(metadataOptions));
at PaintDotNet.Interop.InteropErrorInfo.ThrowIfError() in D:\src\pdn\src\Base\Interop\InteropErrorInfo.cs:line 98
at PaintDotNet.SystemLayer.Native.x64.NativeUtilities.ThrowOnErrorImpl(Int32 hr, SByte* szHr, IUnknown* pFxErrorInfo)
at PaintDotNet.SystemLayer.Native.x64.Imaging.WICImagingFactory.CreateDecoderFromStream(Stream stream, BitmapDecodeOptions metadataOptions, Nullable`1 containerFormat, Nullable`1 preferredVendor)
at PaintDotNet.Data.WicFileTypeHelpers.Load(Stream input, Nullable`1 forcedContainerFormat, Nullable`1 preferredVendor, MetadataTranscoder metadataTranscoder) in D:\src\pdn\src\PaintDotNet\Data\WicFileTypeHelpers.cs:line 61
at PaintDotNet.Data.BmpFileType.OnLoad(Stream input) in D:\src\pdn\src\PaintDotNet\Data\BmpFileType.cs:line 93
at PaintDotNet.FileType.Load(Stream input) in D:\src\pdn\src\Data\FileType.cs:line 482
at PaintDotNet.Functional.Func.Eval[T1,TRet](Func`2 f, T1 arg1) in D:\src\pdn\src\Base\Functional\Func.cs:line 158"
I believe (but not sure) that in previous versions, it would open the file without complaint or (apparent) issue.