diff=astextplain "unsupported filetype" error on mixed-case extension

[KalleOlaviNiemitalo]

Because core.ignorecase defaults to true on Windows, the *.rtf diff=astextplain line in /etc/gitattributes also applies to *.Rtf files with a mixed-case extension; but /usr/bin/astextplain does not recognize the mixed-case extension and fails.

Found by cloning https://github.com/microsoft/Windows-classic-samples and attempting use git log -S. For example:

$ git log -SCreateRemoteThread 2b94df5730177ec27e726b60017c01c97ef1a8fb -- Samples/Win7Samples/winbase/winnt/perftool
E: unsupported filetype C:\Users\Kalle\AppData\Local\Temp/git-blob-a23156/StatList.Rtf
fatal: unable to read files to diff

In this case, the error is due to Samples/Win7Samples/winbase/winnt/perftool/pdh/statlist/StatList.Rtf.

Workarounds:

• git -c core.ignorecase=false, to prevent *.rtf from matching StatList.Rtf
• add *.rtf !diff to .git/info/attributes, to disable astextplain for RTF files altogether

This is git version 2.49.0.windows.1.