Cannot preview Powerpoint file with Uppercase PPTX Extension

Problem

Cannot convert powerpoint file with uppercase PPTX extension

Environment

Confluence Server version 7.15.0

Steps to Reproduce

• Install a fresh Confluence Server version 7.15.0
  • Increase the maximum Heap Size to Xmx 16g and add-in system property -Dconfluence.document.conversion.slides.convert.timeout=300 to prevent bug report CONFSERVER-61143
  • If running Confluence Data Center also addin -Ddocument.conversion.sandbox.disable=true to fallback to Confluence Server type of conversion (not using sandbox)
• Create a new Confluence page and attach the 2 files slide.PPTX and slide2.pptx
• Modify the storage format of the page to the below:

  <p><ac:link><ri:attachment ri:filename="slide.PPTX" /></ac:link></p>
  <p><ac:structured-macro ac:name="viewppt" ac:schema-version="1" ac:macro-id="fe136305-1167-42b2-ae36-ddbc8c893c3d"><ac:parameter ac:name="name"><ri:attachment ri:filename="slide.PPTX" /></ac:parameter></ac:structured-macro></p>
  <p><br /></p>
  <p><ac:link><ri:attachment ri:filename="slide2.pptx" /></ac:link></p>
  <p><ac:structured-macro ac:name="viewppt" ac:schema-version="1" ac:macro-id="13fe90fd-8984-437a-bb78-6ff34e80d60b"><ac:parameter ac:name="name"><ri:attachment ri:filename="slide2.pptx" /></ac:parameter></ac:structured-macro></p>

• Save and Preview the page

Expected Results

• Both previews for slide.PPTX and slide2.pptx should be displayed correctly

Actual Results

• Only the preview for slide2.pptx is working
  • A screenshot is attached to illustrate the behavior

Workaround

Renaming the file to lowercase extension will prevent this issue from happening

Notes

When previewing on "Office Powerpoint"'s macro edit dialog, the preview for slide.PPTX works. But when the page is saved the preview doesn't work anymore.