Steps to reproduce the issue

• Prepare a file and have an uppercase file extension eg sample.PNG
• System > Site Templates (or Administrator Templates)
• Click on template to modify template files
• Click New File to upload the new file to the template

Expected result

Uploaded file appears in the template folder tree so you can further perform actions on the file.

Actual result

File is uploaded, and success message is displayed, but the file does not appear in the folder tree.

Additional comments

Occurs with any file type where the extension is uppercase.

Though you can add the upper case extensions to your list of allowed extensions to fix it, this is not necessarily the best solution, and instead perhaps the extension needs to be converted to lower case on upload?

Discovered while testing #33087