Files downloaded as .bin instead of .pdf

We have made an app wrapper for our internal employee site. Overall it works well, but there are issues with file downloads.

Files are generated by the server and returned as ASP.NET FileContentResult. The content, content type, and content disposition headers are correctly set. I inspected them in the browser. The downloads work in all other browsers, both on the desktop and mobile, but not in the AppMySite app, they get downloaded as .bin files instead of .pdf, for example.

Here are the headers:

Inline content disposition is used to open the file in the browser instead of downloading.

Has anyone else had this issue? Is there any solution?