Hi Mia,
You can try copy the .swf files from Flash to computer local drive and then upload them to the SharePoint online.
Please check if the .swf file has reached the 10 GB file size limits when uploading to SharePoint Online site. Please refer to "File size limits" section in Upload video, audio, or pictures to an Asset Library.
Besides, you can fist zip the file and then try to upload them to SharePoint online to check the outcome.
If the issue persists, please let us know the following information so we can assist you further:
- What happened when you upload the .swf files and the exact place you upload the files, SharePoint library or OneDrive library.
- The screenshots of the error messages and the scenario.
- Your computer system.
[Updated by Jared Cheng MSFT, 7:47 AM, Nov 24, 2016 (UTC)]
Regards,
Jared