I'll improve the error output but in the meantime the error we're getting is as you suspected:
Unexpected char 0xf8 at 36 in Content-Disposition value: form-data; name="ipa"; filename="d-føtex ud af huset.apk"
Which points to the ø character. However this is at the point where we try to upload the resource to AppCenter. Whilst this file is correct and valid on the file system the upload mechanism doesn't like it .
At the moment I am hoping to find a solution that doesn't force users to use only ascii characters but that depends on the http client library that I am using. There is a discussion about a possible solution in this OkHttp issue. That's pretty much your issue right there and it says there is a fix if we bump the version of the library. Hopefully it is as simple as that...... however I'm still testing that.
OK that did work. However the file that you then download from AppCenter has had its name changed from d-føtex ud af huset.apk to d-f_tex_ud_af_huset.apk. I can't quite tell what is doing that. The upgraded library or AppCenter itself. Either way I don't think it matters. You can upload your file!
I'll see if I can provide some tests and then push this to a PR. Thanks again.