Using the new Mail drop feature in Yosemite, you can send an attachment of up to 5GB. Once you want to send an attachment larger than 20MB then the attachment is automatically uploaded to iCloud. With the following terminal command it’s possible to adjust downward the minimum file size of 20MB. As an example we use 10MB, launch Terminal and enter the command.
defaults write com.apple.mail minSizeKB 10000
change 10000 to a different value (5000 for 5MB, 1000 for 1MB,…)
Revert to the default setting:
defaults write com.apple.mail minSizeKB 20000