permits png input for count_objects tool

[kostrykin]

FOR CONTRIBUTOR:

• [x] - I have read the CONTRIBUTING.md document and this tool is appropriate for the tools-iuc repo.
• [x] - License permits unrestricted use (educational + commercial)
• [ ] - This PR adds a new tool or tool collection
• [x] - This PR updates an existing tool or tool collection
• [ ] - This PR does something else (explain below)