It could be the size of the file or the length of the filename.
If you are still unable to attach your document to the application, you can upload it separately to your Applicant Profile. All documents that are uploaded to your Applicant Profile are automatically attached to all applications (previous and future).
To upload documents to your Applicant Profile, follow the instructions below:
From within the Careers page, at the top, click on My Activities, then click the Add Attachment button at the bottom of the page. Select Attachment Type, enter the Attachment Title and click Upload Attachment. Click Choose File, find the file and click Upload.