While a I am editing a page, when I try to include a new image, I can't upload a new file.

Edit->insert->image->attache files->Upload image

If the file is very small (few KB) no error message appears. Instead, when I try to upload large image files, I get the message "Could not upload the file to Confluence. The server may be unavailable."

Note that when I upload images using Tools->Attachments->Attach File, it works just fine for the same images.