The attachment upload error message of the maximum file size should inform the user earlier.

When a file is uploaded via drag and drop the server validates that the size is below the max allowable size and lets you know in a dialog if it will fail or pass.

When using a file chooser the server begins to upload the file but cannot check the size of the file being uploaded until it is at the server.

Validation of total file size should occur before there upload process even starts.