Calling SharePoint Online REST API returns ? instead of umlaut within the data

[John Seto]

When I call SharePoint Online REST API to lookup a list item any values returns that contain umlauts (ä ö ü) are substituted for questions marks (?)

this is the URL i tried directly in the browser?

e.g. https://<tenant>.sharepoint.com/sites/<sitename>/_api/lists/getbytitle('<listname>')/items?$select=Title,Id,Hashcode,Filename,Title/Title&$filter=(guid eq '62f0ff31-3d20-478a-a0d7-19f2d5709e71')

I get this in the Filename - EU-Konformitätserklärung_de,en.pdf is returned as EU-Konformit?tserkl?rung_de,en.pdf

What am I missing?

[#2]

@John Seto 

I´ve tried to reconstruct this in my environment (swedish) but for me it works fine. What kind of field type are you using for FileName? 

[#3]

@Maggan Wåhlin sorry for the late reply, I was expecting an email if anyone replies to me.

re: the field type - "Single line of text"