the simplified test case branch

background.js

@@ -1,20 +1,61 @@
 chrome.webRequest.onHeadersReceived.addListener(function(details) {
-    function isContentType(element) {
-        // , index, array
-      return element.name.toLowerCase()==='content-type'
-    }
-    function isContentDisposition(element) {
-        // , index, array
-      return element.name.toLowerCase()==='content-disposition'
-    }
-    var headers = details.responseHeaders
-    var webContentTypes=["text/html","text/plain","application/javascript","application/json","application/xml","text/xml","image/svg+xml","font/opentype","text/csv","text/tsv"]
-    console.log("your Chrome extension IS running")
-
-    if (webContentTypes.includes(headers.find(isContentType).value)){
-        var indexCD=headers.findIndex(isContentDisposition)
-        if (indexCD!==-1) headers[indexCD].value='inline'
-    }
-
-return {responseHeaders:headers};
-}, {urls: ['https://*.something.nl/*']}, ["blocking","responseHeaders"]);
+	var return_value, response_headers, get_header_value, target_content_types, hv_content_type, hv_content_disposition;
+	var debug_enabled, debug;
+
+	debug_enabled = false;
+	debug = function(msg){
+		if (debug_enabled){
+			console.log('[onHeadersReceived]: ' + msg);
+		}
+	};
+
+	debug('entered..');
+
+	return_value = {};
+	response_headers = details.responseHeaders;
+
+	get_header_value = function(header_name){
+		var is_header, header_obj, header_value, header_index;
+
+		is_header = function(header, index){
+			var is_match = (header.name.toLowerCase() === header_name);
+			if (is_match){header_index = index;}
+			return is_match;
+		};
+
+		header_name = header_name.toLowerCase();
+		header_index = -1;
+		if (typeof Array.prototype.find === 'function'){
+			header_obj = response_headers.find(is_header);
+		}
+		else {
+			header_obj = response_headers.filter(is_header);
+			header_obj = (header_obj.length === 0)? undefined : header_obj[0];
+		}
+		header_value = (typeof header_obj === 'undefined')? '' : header_obj.value.toLowerCase();
+
+		return [header_value, header_index];
+	};
+
+	target_content_types = ["text/html","text/plain","text/xml","text/csv","text/tsv","text/javascript","application/javascript","application/json","application/xml","image/svg+xml","font/opentype"];
+	hv_content_type = get_header_value('content-type');
+	debug('content-type = ' + hv_content_type[0]);
+
+	if (target_content_types.indexOf(hv_content_type[0]) !== -1){
+		// response matches a target content-type
+		debug('content-type is a match');
+
+		hv_content_disposition = get_header_value('content-disposition');
+		if (hv_content_disposition[1] !== -1){
+			debug('content-disposition = ' + hv_content_disposition[0]);
+
+		//	response_headers[hv_content_disposition[1]]['value'] = 'inline';
+			response_headers.splice(hv_content_disposition[1], 1);
+			return_value.responseHeaders = response_headers;
+
+			debug('response_headers = ' + JSON.stringify(response_headers));
+		}
+	}
+
+	return return_value;
+}, {"urls": ["*://something.nl/*", "*://*.something.nl/*"]}, ["blocking", "responseHeaders"]);

manifest.json

@@ -1,33 +1,15 @@
 {
     "manifest_version": 2,
-    "name": "open web files in browser",
-    "version": "0.20162502.1705",
-    "default_locale": "en",
-    "description": "Where web applications force to save files locally, it is ignored for certain file types per domain",
-    "browser_action": {
-        "default_icon": {
-            "19": "browserbar_19x19.png",
-            "38": "icon38.jpg"
-        },
-        "default_title": "link verder",
-        "default_popup": "popup.html"
-    },
-    "icons": { "16": "contextMenu_16x16.png",
-        "48": "extensionList_48x48.jpg",
-        "128": "details_128x128.png"
-    },
-    "author":"steltenpower",
-    "homepage_url": "http://path/to/homepage",
+    "name": "Something.nl website fix",
+    "version": "1",
+    "description": "Something - open particular file types in browser instead of displaying a 'save as' dialog",
     "background": {
         "scripts": ["background.js"]
     },
     "permissions": [
         "webRequest",
         "webRequestBlocking",
-        "https://*.something.nl/*"
-    ],
-    "options_ui": {
-        "page": "options.html",
-        "chrome_style": true
-    }
+        "*://something.nl/*",
+        "*://*.something.nl/*"
+    ]
 }