I needed to make a complaint about an item the other day and using the company's own contact form on my Macbook, it invited me to upload an image. I took a photo of the offending item with my phone and this duly appeared on my Macbook via iCloud. I chose this picture for the form but on completion of the form it said the file uploaded was not an image. I tried a couple of times more with no success. I then tried completing the complaint form on my phone and when it asked for an image to be uploaded I chose the same photo from the phone's version of Photos and this time it worked. In both cases I used the relevant version of Safari. So why should the photo be accepted as an image on the phone but the same photo not be accepted as an image on the Macbook?
My guess would be that the image was in the (relatively) new format HEIC which the site did not understand. This would be transferred to your Mac in that format but when the phone uploaded it, it probably automatically changed it to a JPG which the site could understand.
I've seen some Apple apps creating images with .jpeg file extension instead of Windows expected 3-letter extensions. If the server you are uploading to is Windows, this might be the reason.
I think either of the suggestions could be right I shall have to make a particular note if this happens again. Nevertheless the email complete with photos reached its destination and my complaint was resolved satisfactorily.
Lionel Ogden 