Issues with uploading files to iCloud Photo on PC Win10

Windows 10 laptop. Plenty of storage. Good internet connection. Have checked multiple similar questions and answers on Community however 99% are the usual useless answers like "make sure you're connected to the net".

I am attempting to move photos and video from MSFT OneDrive (files held locally on the laptop) to iCloud Photos.

(1) After adding the iCloud Photos for Windows app to the laptop I created folders in iCloud Windows via File Explorer. I then copied and pasted the files from OneDrive to iCloud Photos in File Explorer. Some were uploaded, some not, despite being acceptable file types JPEG, PNG, GIF, TIFF, H.264, HEVC, MPEG-4

(2) Dragging and dropping files from OneDrive to iCloud Photos web access - error is "Only files in JPEG format can be uploaded" even though some of the files are MPEG-4.

I understand from this Community there are many people with similar issues. Does anyone have a real solution?

Thanks in advance.