Patching libchromiumcontent for dragOut. Patched against 0fbded6c of libchromiumcontent: https://github.com/atom/libchromiumcontent/commit/0fbded6cf3d9244389db05f0c022e474a06ad32a

A_Desc.js

// We use a magic string to hook into the functionality

foo.addEventListener('dragstart', function( e ) {
  
  var paths = '/Foo/Bar/baz.jpg|\n|/Foo/Baz/Bar.jpg';
  
  e.dataTransfer.setData( 'text/plain',
                          'extensis-filenames-type:' + paths );
                          
});

web_contents_view_aura.cc.patch

diff --git a/content/browser/web_contents/web_contents_view_aura.cc b/content/browser/web_contents/web_contents_view_aura.cc
index e6c144b..48b2119 100644
--- a/content/browser/web_contents/web_contents_view_aura.cc
+++ b/content/browser/web_contents/web_contents_view_aura.cc
@@ -4,6 +4,11 @@

 #include "content/browser/web_contents/web_contents_view_aura.h"

+#include <windows.h>
+#include <string>
+#include <shlobj.h>
+#include "ObjIdl.h"
+
 #include "base/auto_reset.h"
 #include "base/command_line.h"
 #include "base/files/file_util.h"
@@ -68,6 +73,13 @@
 #include "ui/wm/public/drag_drop_delegate.h"

 namespace content {
+
+// BEGIN EXTENSIS
+
+HRESULT DragFilesToDesktop(HWND window, std::vector<base::string16> filenames);
+
+// END EXTENSIS
+
 WebContentsView* CreateWebContentsView(
     WebContentsImpl* web_contents,
     WebContentsViewDelegate* delegate,
@@ -271,6 +283,50 @@ void PrepareDragData(const DropData& drop_data,
                      WebContentsImpl* web_contents) {
   provider->MarkOriginatedFromRenderer();
 #if defined(OS_WIN)
+
+  // BEGIN EXTENSIS
+  // Note this is experimental. This will start a drag & drop and exit without
+  // calling any subsequent code. We'll want to see if any of that is needed.
+  if (!drop_data.text.string().empty()) {
+    const base::string16& tmp = drop_data.text.string();
+    const base::char16 kExtensisFilenamesToken[] = { 'e', 'x', 't', 'e', 'n', 's',
+      'i', 's', '-', 'f', 'i', 'l', 'e', 'n', 'a', 'm', 'e', 's', '-', 't', 'y',
+      'p', 'e', ':', 0 };
+    const base::char16 kFilenamesDelimiterToken[] = { '|', '\n', '|', 0 };
+    std::vector<base::string16> filenames;
+
+    if (tmp.find(kExtensisFilenamesToken) == 0) {
+      size_t pos = base::string16(kExtensisFilenamesToken).length();
+      while (pos != base::string16::npos) {
+        base::string16 filename;
+        size_t pos2 = tmp.find(kFilenamesDelimiterToken, pos);
+
+        if (pos2 == base::string16::npos) {
+          filename = tmp.substr(pos);
+          filenames.push_back(filename);
+          pos = pos2;
+        } else {
+          filename = tmp.substr(pos, pos2 - pos);
+          filenames.push_back(filename);
+          pos = pos2 + base::string16(kFilenamesDelimiterToken).length();
+        }
+      }
+
+      HWND native_window = web_contents->GetView()->GetNativeView()->GetHost()->GetAcceleratedWidget();
+      HRESULT hr = DragFilesToDesktop(native_window, filenames);
+
+      if(SUCCEEDED(hr)) {
+        // Abort default d&d behavior, it was already handled.
+        return;
+      } else {
+        // Continue with default d&d behavior. This will drag thumbnail images
+        // for selected items. TODO We may wish to display an error message
+        // instead.
+      }
+    }
+  }
+  // END EXTENSIS
+
   // Put download before file contents to prefer the download of a image over
   // its thumbnail link.
   if (!drop_data.download_metadata.empty())
@@ -1322,4 +1378,303 @@ void WebContentsViewAura::UpdateWebContentsVisibility(bool visible) {
   }
 }

+// BEGIN EXTENSIS
+
+class CExtDropSource : public IDropSource
+{
+public:
+  // *** IUnknown ***
+  STDMETHODIMP QueryInterface(REFIID riid, void **ppv);
+  STDMETHODIMP_(ULONG) AddRef();
+  STDMETHODIMP_(ULONG) Release();
+
+  // *** IDropSource ***
+  STDMETHODIMP QueryContinueDrag(BOOL fEscapePressed, DWORD grfKeyState);
+  STDMETHODIMP GiveFeedback(DWORD dwEffect);
+
+  CExtDropSource() : m_refCount(1) { }
+private:
+  LONG m_refCount;
+};
+
+HRESULT CExtDropSource::QueryInterface(REFIID riid, void **ppv)
+{
+  IUnknown *punk = NULL;
+  if (riid == IID_IUnknown) {
+    punk = static_cast<IUnknown*>(this);
+  } else if (riid == IID_IDropSource) {
+    punk = static_cast<IDropSource*>(this);
+  }
+
+  *ppv = punk;
+  if (punk) {
+    punk->AddRef();
+    return S_OK;
+  } else {
+    return E_NOINTERFACE;
+  }
+}
+
+ULONG CExtDropSource::AddRef(void) {
+  // increment object reference count
+  return InterlockedIncrement(&m_refCount);
+}
+
+ULONG CExtDropSource::Release(void) {
+  // decrement object reference count
+  LONG count = InterlockedDecrement(&m_refCount);
+
+  if(count == 0) {
+    delete this;
+    return 0;
+  }
+  else {
+    return count;
+  }
+}
+
+HRESULT CExtDropSource::QueryContinueDrag(
+  BOOL fEscapePressed, DWORD grfKeyState)
+{
+  if (fEscapePressed) return DRAGDROP_S_CANCEL;
+
+  if (!(grfKeyState & (MK_LBUTTON | MK_RBUTTON))) {
+    return DRAGDROP_S_DROP;
+  }
+
+  return S_OK;
+}
+
+HRESULT CExtDropSource::GiveFeedback(DWORD dwEffect)
+{
+  return DRAGDROP_S_USEDEFAULTCURSORS;
+}
+
+// Implement IDataObject for drag & drop of files to desktop and applications
+class CExtDataObject : public IDataObject {
+public:
+  // IUnknown members
+  STDMETHODIMP      QueryInterface (REFIID iid, void ** ppvObject);
+  STDMETHODIMP_(ULONG)  AddRef (void);
+  STDMETHODIMP_(ULONG)  Release (void);
+
+  // IDataObject members
+  STDMETHODIMP GetData        (FORMATETC *formatEtc,  STGMEDIUM *pMedium);
+  STDMETHODIMP GetDataHere      (FORMATETC *formatEtc,  STGMEDIUM *pMedium);
+  STDMETHODIMP QueryGetData     (FORMATETC *formatEtc);
+  STDMETHODIMP GetCanonicalFormatEtc  (FORMATETC *formatEtc,  FORMATETC *pFormatEtcOut);
+  STDMETHODIMP SetData        (FORMATETC *formatEtc,  STGMEDIUM *pMedium,  BOOL fRelease);
+  STDMETHODIMP EnumFormatEtc      (DWORD      dwDirection, IEnumFORMATETC **ppEnumFormatEtc);
+  STDMETHODIMP DAdvise        (FORMATETC *formatEtc,  DWORD advf, IAdviseSink *pAdvSink, DWORD *pdwConnection);
+  STDMETHODIMP DUnadvise        (DWORD      dwConnection);
+  STDMETHODIMP EnumDAdvise      (IEnumSTATDATA **ppEnumAdvise);
+
+  // Constructor / Destructor
+  CExtDataObject(std::vector<base::string16> paths);
+  ~CExtDataObject();
+
+private:
+
+  int LookupFormatEtc(FORMATETC *pFormatEtc);
+  HGLOBAL CreateDropfiles(std::vector<base::string16>& paths);
+
+  std::vector<base::string16> m_paths;
+  LONG     m_refCount;
+  FORMATETC  m_formatEtc;
+};
+
+CExtDataObject::CExtDataObject(std::vector<base::string16> paths) {
+
+  m_paths = paths;
+
+  // reference count must ALWAYS start at 1
+  m_refCount    = 1;
+
+  m_formatEtc.cfFormat = CF_HDROP;
+  m_formatEtc.dwAspect = DVASPECT_CONTENT;
+  m_formatEtc.lindex = -1;
+  m_formatEtc.ptd = NULL;
+  m_formatEtc.tymed = TYMED_HGLOBAL;
+}
+
+CExtDataObject::~CExtDataObject() {
+
+}
+
+ULONG CExtDataObject::AddRef(void) {
+  // increment object reference count
+  return InterlockedIncrement(&m_refCount);
+}
+
+ULONG CExtDataObject::Release(void) {
+  // decrement object reference count
+  LONG count = InterlockedDecrement(&m_refCount);
+
+  if(count == 0) {
+    delete this;
+    return 0;
+  }
+  else {
+    return count;
+  }
+}
+
+HRESULT CExtDataObject::QueryInterface(REFIID iid, void **ppvObject) {
+  // check to see what interface has been requested
+  if(iid == IID_IDataObject || iid == IID_IUnknown) {
+    AddRef();
+    *ppvObject = this;
+    return S_OK;
+  } else {
+    *ppvObject = 0;
+    return E_NOINTERFACE;
+  }
+}
+
+HRESULT CExtDataObject::QueryGetData(FORMATETC *formatEtc) {
+  return (LookupFormatEtc(formatEtc) == -1) ? DV_E_FORMATETC : S_OK;
+}
+
+int CExtDataObject::LookupFormatEtc(FORMATETC *formatEtc) {
+  if((m_formatEtc.tymed    &  formatEtc->tymed)   &&
+    m_formatEtc.cfFormat == formatEtc->cfFormat &&
+    m_formatEtc.dwAspect == formatEtc->dwAspect) {
+    return 0;
+  }
+
+  // error, format not found
+  return -1;
+}
+
+HRESULT CExtDataObject::GetData(FORMATETC *formatEtc, STGMEDIUM *stgMedium) {
+  std::vector<base::string16> pathsThatExist;
+
+  // try to match the requested FORMATETC with one of our supported formats
+  if(LookupFormatEtc(formatEtc) == -1) {
+    return DV_E_FORMATETC;
+  }
+
+  stgMedium->tymed = m_formatEtc.tymed;
+  stgMedium->pUnkForRelease = 0;
+
+  switch(m_formatEtc.tymed) {
+  case TYMED_HGLOBAL:
+
+    // Create DROPFILES just in time, using only files that actually exist
+    for(std::vector<base::string16>::iterator i = m_paths.begin(); i != m_paths.end(); i++) {
+      if(INVALID_FILE_ATTRIBUTES != GetFileAttributes(i->c_str())) {
+        pathsThatExist.push_back(*i);
+      }
+    }
+
+    stgMedium->hGlobal = CreateDropfiles(pathsThatExist);
+    break;
+
+  default:
+    return DV_E_FORMATETC;
+  }
+
+  return S_OK;
+}
+
+HRESULT CExtDataObject::GetDataHere(FORMATETC *pFormatEtc, STGMEDIUM *pMedium) {
+  // GetDataHere is only required for IStream and IStorage mediums
+  return DATA_E_FORMATETC;
+}
+
+HRESULT CExtDataObject::EnumFormatEtc(DWORD dwDirection, IEnumFORMATETC **ppEnumFormatEtc) {
+  if(dwDirection == DATADIR_GET) {
+    return SHCreateStdEnumFmtEtc(1, &m_formatEtc, ppEnumFormatEtc);
+  } else {
+    // the direction specified is not support for drag+drop
+    return E_NOTIMPL;
+  }
+}
+
+HRESULT CExtDataObject::DAdvise (FORMATETC *pFormatEtc, DWORD advf, IAdviseSink *pAdvSink, DWORD *pdwConnection) {
+  return OLE_E_ADVISENOTSUPPORTED;
+}
+
+HRESULT CExtDataObject::DUnadvise(DWORD dwConnection) {
+  return OLE_E_ADVISENOTSUPPORTED;
+}
+
+HRESULT CExtDataObject::EnumDAdvise(IEnumSTATDATA **ppEnumAdvise) {
+  return OLE_E_ADVISENOTSUPPORTED;
+}
+
+HRESULT CExtDataObject::GetCanonicalFormatEtc(FORMATETC *pFormatEct, FORMATETC *formatEtcOut) {
+  // Apparently we have to set this field to NULL even though we don't do anything else
+  formatEtcOut->ptd = NULL;
+  return E_NOTIMPL;
+}
+
+HRESULT CExtDataObject::SetData (FORMATETC *pFormatEtc, STGMEDIUM *pMedium,  BOOL fRelease) {
+  return E_NOTIMPL;
+}
+
+HGLOBAL CExtDataObject::CreateDropfiles(std::vector<base::string16>& paths) {
+
+  if (paths.empty()) {
+    return NULL;
+  }
+
+  // The DROPFILES struct will be followed by a double-null-terminated list of strings containing file paths
+  // Determine the memory needed to hold the struct plus the files
+  size_t bufSize = sizeof(DROPFILES);
+  for(std::vector<base::string16>::iterator i = paths.begin(); i != paths.end(); i++) {
+    bufSize += (sizeof(WCHAR) * (i->size() + 1));
+  }
+
+  HGLOBAL hMem = GlobalAlloc(GHND, bufSize + sizeof(WCHAR));
+  if (!hMem) {
+    return NULL;
+  }
+
+  DROPFILES *dfiles = (DROPFILES*) GlobalLock(hMem);
+  if (!dfiles) {
+    GlobalFree(hMem);
+    return NULL;
+  }
+
+  dfiles->pFiles = sizeof(DROPFILES);
+  GetCursorPos(&(dfiles->pt));
+  dfiles->fNC = TRUE;
+  dfiles->fWide = TRUE;
+
+  // Copy each path to the end of the structure
+  char *pathBuffer = (char*) &dfiles[1];
+  for(std::vector<base::string16>::iterator i = paths.begin(); i != paths.end(); i++) {
+    size_t len = sizeof(WCHAR) * (i->length() + 1);
+    memcpy(pathBuffer, i->c_str(), len);
+    pathBuffer += len;
+  }
+
+  GlobalUnlock(hMem);
+
+  return hMem;
+}
+
+HRESULT DragFilesToDesktop(HWND window, std::vector<base::string16> paths) {
+  HRESULT hr = S_OK;
+  IDataObject *dataObject = new CExtDataObject(paths);
+
+  if(dataObject) {
+    IDropSource *dropSource = new CExtDropSource();
+    if(dropSource) {
+      DWORD dwEffect = 0;
+      DoDragDrop(dataObject, dropSource, DROPEFFECT_COPY, &dwEffect);
+      dropSource->Release();
+    } else {
+      hr = E_OUTOFMEMORY;
+    }
+
+    dataObject->Release();
+  }
+
+  return hr;
+}
+
+// END EXTENSIS
+
 }  // namespace content

web_drag_source_mac.mm.patch

diff --git a/content/browser/web_contents/web_drag_source_mac.mm b/content/browser/web_contents/web_drag_source_mac.mm
index ff3a881..781d85d 100644
--- a/content/browser/web_contents/web_drag_source_mac.mm
+++ b/content/browser/web_contents/web_drag_source_mac.mm
@@ -444,8 +444,25 @@ void PromiseWriterHelper(const DropData& drop_data,

   // Plain text.
   if (!dropData_->text.string().empty()) {
-    [pasteboard_ addTypes:@[ NSStringPboardType ]
-                    owner:contentsView_];
+    NSString *tmp = SysUTF16ToNSString(dropData_->text.string());
+
+    NSString *kExtensisFilenamesToken = @"extensis-filenames-type:";
+    NSString *kFilenamesDelimiterToken = @"|\n|";
+
+    // If this is a special Extensis kind of text, parse it,
+    // and set a NSFilenamesPboardType pasteboard property.
+    NSRange filenameTokenLoc = [tmp rangeOfString:kExtensisFilenamesToken];
+    if (filenameTokenLoc.location != NSNotFound) {
+      NSString *filenamesStr = [tmp substringFromIndex:filenameTokenLoc.location + filenameTokenLoc.length];
+      NSArray *filenames = [filenamesStr componentsSeparatedByString:kFilenamesDelimiterToken];
+      if ([filenames count] > 0) {
+        dragOperationMask_ &= ~NSDragOperationMove;
+        [pasteboard_ setPropertyList:filenames forType:NSFilenamesPboardType];
+      }
+    } else {
+      [pasteboard_ addTypes:@[ NSStringPboardType ]
+                      owner:contentsView_];
+    }
   }

   if (!dropData_->custom_data.empty()) {