What you could try is: introduce a condition (validator) to your workflow. When your users try to enter the "closed" status, the validator should check if the issue has at least one Excel attachment, and possibly check its filename, etc.
The Script Runner allows writing custom validators in Groovy, and it really should not take more than 10-15 lines of code: https://jamieechlin.atlassian.net/wiki/display/GRV/Validators
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