Admin CP Check for max. Uploadfilesize on Attachment Manager Site
 

You should check for the relevant sizes(PHP Max Post Size, PHP Maximum Upload Size, if stored in database also MySQL Packet Size,..) on editing/adding new Extensions.

For example: If you have a jpg/jpeg file which is allowed to be max. 15mb and the PHP max. Upload Size is set only to 5mb you will get a error in the frontend (there will be max. 15mb displayed)
After the upload it shows that "filesize is too large."

If the Size would be set "right", you wouldn't get this unnecessary message.