diff --git a/lib/getFilterInfosAndTargetContentTypeFromQueryString.js b/lib/getFilterInfosAndTargetContentTypeFromQueryString.js
index 55c1436..5096f33 100644
--- a/lib/getFilterInfosAndTargetContentTypeFromQueryString.js
+++ b/lib/getFilterInfosAndTargetContentTypeFromQueryString.js
@@ -2,6 +2,7 @@
 var Stream = require('stream'),
     _ = require('underscore'),
     gm = require('gm'),
+    mime = require('mime'),
     exifReader = require('exif-reader'),
     icc = require('icc'),
     sharp,
@@ -25,6 +26,15 @@ Object.keys(gm.prototype).forEach(function (propertyName) {
     }
 });
 
+function getMockFileNameForContentType(contentType) {
+    if (contentType) {
+        if (contentType === 'image/vnd.microsoft.icon') {
+            return '.ico';
+        }
+        return mime.extensions[contentType];
+    }
+}
+
 // For compatibility with the sharp format switchers (minus webp, which graphicsmagick doesn't support).
 // Consider adding more from this list: gm convert -list format
 ['jpeg', 'png'].forEach(function (formatName) {
@@ -60,7 +70,7 @@ module.exports = function getFilterInfosAndTargetContentTypeFromQueryString(quer
         operationNames = [],
         usedQueryStringFragments = [],
         leftOverQueryStringFragments = [],
-        targetContentType,
+        targetContentType = options.sourceContentType,
         root = options.root || options.rootPath;
 
     function checkSharpOrGmOperation(operation) {
@@ -115,7 +125,7 @@ module.exports = function getFilterInfosAndTargetContentTypeFromQueryString(quer
                                 spawned = true;
                                 var seenData = false,
                                     hasEnded = false,
-                                    gmInstance = gm(readStream);
+                                    gmInstance = gm(readStream, getMockFileNameForContentType(gmOperationsForThisInstance[0].sourceContentType));
                                 if (options.maxInputPixels) {
                                     gmInstance.limit('pixels', options.maxInputPixels);
                                 }
@@ -188,7 +198,6 @@ module.exports = function getFilterInfosAndTargetContentTypeFromQueryString(quer
             if (filters[operationName]) {
                 flushOperations();
                 filterInfo = filters[operationName](operationArgs, {
-                    inputContentType: targetContentType,
                     numPreceedingFilters: filterInfos.length
                 });
                 if (filterInfo) {
@@ -274,6 +283,7 @@ module.exports = function getFilterInfosAndTargetContentTypeFromQueryString(quer
                             currentEngineName = candidateEngineNames[0];
                         }
                     }
+                    var sourceContentType = targetContentType;
                     if (operationName === 'setFormat' && operationArgs.length > 0) {
                         var targetFormat = operationArgs[0].toLowerCase();
                         if (targetFormat === 'jpg') {
@@ -283,7 +293,7 @@ module.exports = function getFilterInfosAndTargetContentTypeFromQueryString(quer
                     } else if (operationName === 'jpeg' || operationName === 'png' || operationName === 'webp') {
                         targetContentType = 'image/' + operationName;
                     }
-                    operations.push({name: operationName, args: operationArgs, usedQueryStringFragment: keyValuePair});
+                    operations.push({sourceContentType: sourceContentType, name: operationName, args: operationArgs, usedQueryStringFragment: keyValuePair});
                     usedQueryStringFragments.push(keyValuePair);
                 }
             } else {
diff --git a/lib/processImage.js b/lib/processImage.js
index 5e0dc36..813cf63 100644
--- a/lib/processImage.js
+++ b/lib/processImage.js
@@ -72,7 +72,8 @@ module.exports = function (options) {
 
                 if (contentType && contentType.indexOf('image/') === 0) {
                     var filterInfosAndTargetFormat = getFilterInfosAndTargetContentTypeFromQueryString(queryString, _.defaults({
-                            sourceFilePath: options.root && Path.resolve(options.root, req.url.substr(1))
+                            sourceFilePath: options.root && Path.resolve(options.root, req.url.substr(1)),
+                            sourceContentType: contentType
                         }, options)),
                         targetContentType = filterInfosAndTargetFormat.targetContentType;
                     if (filterInfosAndTargetFormat.filterInfos.length === 0) {
diff --git a/test/processImage.js b/test/processImage.js
index cb31812..7c803ab 100644
--- a/test/processImage.js
+++ b/test/processImage.js
@@ -1,5 +1,6 @@
 /*global describe, it, beforeEach, __dirname*/
 var express = require('express'),
+    fs = require('fs'),
     pathModule = require('path'),
     unexpected = require('unexpected'),
     processImage = require('../lib/processImage'),
@@ -262,6 +263,43 @@ describe('express-processimage', function () {
         });
     });
 
+    // Undetectable by gm -- the source format must be explicitly specified
+    it('should convert an icon to png via GraphicsMagick', function () {
+        return expect('GET /favicon.ico?gm&setFormat=png', 'to yield response', {
+            headers: {
+                'Content-Type': 'image/png'
+            },
+            body: expect.it('to have metadata satisfying', {
+                format: 'PNG',
+                size: {
+                    width: 16,
+                    height: 16
+                }
+            })
+        });
+    });
+
+    it('should convert an icon served as image/vnd.microsoft.icon to png via GraphicsMagick', function () {
+        return expect(express().use(processImage(config)).get('/favicon.ico', function (req, res, next) {
+            res.setHeader('Content-Type', 'image/vnd.microsoft.icon');
+            fs.createReadStream(pathModule.resolve(__dirname, '..', 'testdata', 'favicon.ico')).pipe(res);
+        }), 'to yield exchange', {
+            request: 'GET /favicon.ico?gm&setFormat=png',
+            response: {
+                headers: {
+                    'Content-Type': 'image/png'
+                },
+                body: expect.it('to have metadata satisfying', {
+                    format: 'PNG',
+                    size: {
+                        width: 16,
+                        height: 16
+                    }
+                })
+            }
+        });
+    });
+
     describe.skipIf(!sharp, 'when sharp is available', function () {
         it('should allow retrieving the image metadata as JSON', function () {
             return expect('GET /turtle.jpg?metadata', 'to yield response', {