Textual attachments are served in UTF8 even if encoded differently

Steps to reproduce

• Create page and attach to it a textual file encoded in ISO-8859-15 such as this one: test-iso-8859-15.txt
• Visit attachment URL

Actual result

The file content is displayed with invalid characters (see text.png) because its content type is set to UTF8 in the response (as shown in the response content-type header: content-type: text/plain;charset=UTF-8) while it is encoded in ISO-8859-1. When using a file encoded in UTF8, its content is displayed correctly.