When attempting to upload a new version of an attachment (regardless of file size or file type) using either the Image macro > "Upload Image" or the drag and drop functionality, the following error message appears:

Could not upload the file to Confluence. The server may be unavailable.

This error message does not occur when uploading the original attachment.

Attachments can only be uploaded by either removing the original attachment or navigating to Tools > Attachment and then selecting "Lock to Edit."