yahoo mail attachment problem due to various reasons. Attachments having unusual characters in their names, such as slashes, hashtags, or dollar signs, are not accepted by Yahoo Mail. Additionally, Yahoo likewise sets a limit of 25MB for overall message size. 

Yahoo also believes that the encoding method it uses to enable other email service providers to read your email can increase the size by about 33%, so your file can't be that large. You can't attach encrypted files either, so if you're worried about prying eyes, you're out of luck.