I'm trying to add an attachment in Outlook to an email message. When I drag and drop the specified file, its file extension is in some cases converted to uppercase. It seems that the last character of the filename without the extension is checked, and if that's an uppercase character, then the extension is converted accordingly.

  • someFile.ext is left unharmed;
  • HTTP.java is changed to HTTP.JAVA;
  • MIT.pdf is changed to MIT.PDF;
  • FileB.sh is changed to FileB.SH.

Why is that happening? Why should Microsoft convert the file extension and not just leave it alone?
And how can I disable this behavior? Or otherwise, is there a workaround available?

  • 2
    This has been bugging me for a while too. Not an answer, so to speak, but more info or a workaround - if you use the "Attach File" dialogue, this extension case conversion doesn't happen. Why the difference ? No idea, but it seems to sort it. – Youra Turceninoff Jul 3 '17 at 9:17
  • It depends on the last letter of the file name, if it end with a capital letter, the file type will be capitalized! – Luca Ziegler Nov 7 '17 at 10:23

I faced this issue recently and could not believe that Outlook changes the extension to upper case if the last character before the . is upper case.

This only happens for the first file attachment, if you attach more files, the extension stays the same.

Check out one of the weirdest bug in Outlook.

Microsoft continues to amaze me!

Outlook Uppercase Extension Bug

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