Major thumbs up on the idea and the simple interface, though some feedback:

If everything is local, using “Download” and “Upload” doesn’t seem like the proper choice of words.

It’s confusing that the Download button is on a table which shows the name of the file we “uploaded”, making it look like we’ll get the same file and not the conversion.

It might be worth considering some basic options to determine quality. Nothing too fancy, but at least an idea of what the quality/size ratio will be. For testing purposes I converted an MP4 to WebM which reduced it to less than one sixth of the size but the quality was so destroyed that the time and resources it took were a waste, the file went straight into the bin.