Lowercase file extensions vs Uppercase file extensions within a SharePoint List

[hitty08]

Currently I am running into an issue within a SharePoint Online list, in an attachments column, documents are opening in the browser if they are named with a lowercase file extension and asking the user to download if they are named with a uppercase file extension. 

Does anyone know why this could be happening (if their is Microsoft documentation on it that would be great)? Right now I can convert all the files to be lowercase, but I am just wondering why this is happening. 

Thank you! 

[#2]

@hitty08 Strange! I am unable to replicate this issue on my SharePoint tenant. I just tried with .xlsx & .XLSX and .docx & .DOCX files and both are opening fine in browser.

Was this working for you before this?

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[#3]

This is the first time I am hear this being an issue and to be honest I have never really payed attention to the Uppercase vs Lowercase extensions because it has never been an issue.

I do think now that I have looked deeper into this it is only with .pdf's

[#4]

@hitty08 This is working for PDF files as well on my SharePoint site. 

1. Can you try creating a new list in same site or another site and check if you can reproduce the same?
2. Which browser are you using?

Check this thread for browser settings for PDF files if it helps: How to change browser download settings for PDF files 

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[#5]

@hitty08 : How can you convert the uppercase to lowercase easy for all files in a Sharepoint site?