diff --git a/lib/http-api.js b/lib/http-api.js
index 3dd86615c..d771471f7 100644
--- a/lib/http-api.js
+++ b/lib/http-api.js
@@ -207,6 +207,10 @@ module.exports.MatrixHttpApi.prototype = {
             xhr.open("POST", url);
             if (file.type) {
                 xhr.setRequestHeader("Content-Type", file.type);
+            } else {
+                // if the file doesn't have a mime type, use a default since
+                // the HS errors if we don't supply one.
+                xhr.setRequestHeader("Content-Type", 'application/octet-stream');
             }
             xhr.send(file);
         } else {