It tries to show you a preview of what you're uploading. However, web browsers support only a few very specific formats, such as MPEG4 with H.264/H.265 + AAC, Ogg with Theora+Vorbis, and WebM with VP8/VP9 + Vorbis/Opus. If your browser doesn't support the combination you give it, it won't be able to show it in the preview. However, DTube will most likely still be able to process the file to generate the 480p version. Viewers probably won't be able to watch the original for the above reason, though they should be able to download it.