Remove validation on mta extension file name

[tal-sapan]

Other tools (e.g. mta deployer) don't check the file name, so there is no need to limit it.

Checklist

• [X] Code compiles correctly
• [X] Relevant tests were added (unit / contract / integration)
• [ ] Relevant logs were added
• [X] Formatting and linting run locally successfully
• [X] All tests passing
• [ ] UA review
• [ ] Design is documented
• [ ] Extended the README / documentation, if necessary
• [ ] Open source is approved