Unable to upload a PNG to include in a post from local computer

Question

It would seem it is not currently possible to upload a PNG image from my local file system:

This happens when I try to upload any PNG. I just had to first upload to ImageShack and then upload to Stack Overflow via the "From the web" option.

Despite the fact that I am using IE8, I don't believe this to be browser-related, because I can upload GIFs and JPEGs with no problem.

The headers being sent by the browser:

POST http://meta.stackoverflow.com/upload/image HTTP/1.1
Accept: application/x-ms-application, image/jpeg, application/xaml+xml, image/gif, image/pjpeg, application/x-ms-xbap, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*
Referer: http://meta.stackexchange.com/questions/184641/unable-to-upload-a-png-to-include-in-a-post-from-local-computer
Accept-Language: ru-RU
User-Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; MAPB; .NET4.0C; .NET4.0E; .NET CLR 1.1.4322)
Content-Type: multipart/form-data; boundary=---------------------------7dd2b2931053c
Accept-Encoding: gzip, deflate
Host: meta.stackoverflow.com
Content-Length: 37481
Connection: Keep-Alive
Pragma: no-cache
Cookie: wonttellya

-----------------------------7dd2b2931053c
Content-Disposition: form-data; name="fkey"

f2fbecdc7ac0c6592e7aa80bcf54f345
-----------------------------7dd2b2931053c
Content-Disposition: form-data; name="source"

computer
-----------------------------7dd2b2931053c
Content-Disposition: form-data; name="filename"; filename="S6lQ7.png"
Content-Type: image/x-png

Answer 1

In our image upload backend, we are checking for known mime-types to ensure that the content being uploaded is indeed an image.

image/x-png was not on the list.

It will be in the next build.