I can’t tell for sure without seeing your whole request but it looks like you’re trying to pass just a filename. As @Bastien_Siebman says above, you need to pass the binary contents of the file.
You might find these forum posts helpful:
In terms of testing, I’m also not familiar with Insomenia but Asana provides a full collection of samples of their API calls for use in Postman here, including the “Upload an attachment” endpoint: