Cache full: When your cache is full, this may cause Yahoo! Mail not to open your file attachments correctly. To fix this problem, sign out of your account first, and then clear your PC cache. To clear your cache, click on the Tools menu and select "Delete Temporary Internet files" option in order to clear your cache.
Page in idle state: If you have logged in for quite a long time, then it might be that your Yahoo! Mail page has gone in an idle state. Consider reloading the page and then retry attaching your files.
Corrupt attachment files: Check to ensure that the file attachments are not corrupt. The file attachments may corrupt if there was an error in the process of uploading the files or if the files were not fully uploaded before they were sent.
Unsupported file format: Ensure that the attachment file format is supported for Yahoo download. Some files formats cannot open unless you install specific programs to help in opening them.