The file size error messages for max_image_size_kb and max_attachment_size_kb are shown to the user in the KB format, regardless of how large the limit is. Since we are going to support uploading much larger files soon, this KB-based limit soon becomes unfriendly to the end user.
For example, if the max attachment size is set to 512000 KB, this is what the user sees:
Sorry, the file you are trying to upload is too big (maximum size is 512000KB)
This makes the user do math. In almost all file explorers that a regular user would be familiar width, the file size is shown in a format based on the maximum increment (e.g. KB, MB, GB).
This commit changes the behaviour to output a humanized file size instead of the raw KB. For the above example, it would now say:
Sorry, the file you are trying to upload is too big (maximum size is 512 MB)
This humanization also handles decimals, e.g. 1536KB = 1.5 MB