This is usually due to characters in the file name.
File names can only contain a-z, A-Z, 0-9, - _ ( ) & + = . , ' and the space character. Any other characters will prevent the attachment from coming through into Mosaic.
This change has been in effect since version 22.2 of Mosaic.
If the above doesn't work, please raise a new case online and reference the title of this article.