Email fails to process on the Incoming Mail Handler when the attachment is over 10MB.
The incoming email size (14596668 bytes) exceeds the maximum allowed message size (10485760 bytes).
You can setup a forward email to forward the error message onto another email but it still won't be processed as an issue Jira.
Allow the issue to be created, ignoring the attachment if it's too large but add a warning message onto the issue saying it failed to upload an attachment.
Some client's are left in the dark to whether they have successfully created an issue or not.