diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/PackTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/PackTest.java
index 85bd31d..a359654 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/PackTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/PackTest.java
@@ -261,7 +261,7 @@
 				new PackIndexWriterV1(f).write(list, footer);
 			}
 
-			Pack pack = new Pack(packName, PackExt.INDEX.getBit());
+			Pack pack = new Pack(packName, null);
 			try {
 				pack.get(wc, b);
 				fail("expected LargeObjectException.ExceedsByteArrayLimit");
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectory.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectory.java
index e71a960..170df57 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectory.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectory.java
@@ -11,8 +11,8 @@
 package org.eclipse.jgit.internal.storage.file;
 
 import static java.nio.charset.StandardCharsets.UTF_8;
-import static org.eclipse.jgit.internal.storage.pack.PackExt.INDEX;
 import static org.eclipse.jgit.internal.storage.pack.PackExt.PACK;
+import static org.eclipse.jgit.internal.storage.pack.PackExt.BITMAP_INDEX;
 
 import java.io.BufferedReader;
 import java.io.File;
@@ -31,7 +31,6 @@
 
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.internal.storage.pack.ObjectToPack;
-import org.eclipse.jgit.internal.storage.pack.PackExt;
 import org.eclipse.jgit.internal.storage.pack.PackWriter;
 import org.eclipse.jgit.lib.AbbreviatedObjectId;
 import org.eclipse.jgit.lib.AnyObjectId;
@@ -216,26 +215,26 @@
 	 * Add a single existing pack to the list of available pack files.
 	 */
 	@Override
-	public Pack openPack(File pack)
-			throws IOException {
-		final String p = pack.getName();
-		if (p.length() != 50 || !p.startsWith("pack-") || !p.endsWith(".pack")) //$NON-NLS-1$ //$NON-NLS-2$
-			throw new IOException(MessageFormat.format(JGitText.get().notAValidPack, pack));
-
-		// The pack and index are assumed to exist. The existence of other
-		// extensions needs to be explicitly checked.
-		//
-		int extensions = PACK.getBit() | INDEX.getBit();
-		final String base = p.substring(0, p.length() - 4);
-		for (PackExt ext : PackExt.values()) {
-			if ((extensions & ext.getBit()) == 0) {
-				final String name = base + ext.getExtension();
-				if (new File(pack.getParentFile(), name).exists())
-					extensions |= ext.getBit();
-			}
+	public Pack openPack(File pack) throws IOException {
+		PackFile pf;
+		try {
+			pf = new PackFile(pack);
+		} catch (IllegalArgumentException e) {
+			throw new IOException(
+					MessageFormat.format(JGitText.get().notAValidPack, pack),
+					e);
 		}
 
-		Pack res = new Pack(pack, extensions);
+		String p = pf.getName();
+		// TODO(nasserg): See if PackFile can do these checks instead
+		if (p.length() != 50 || !p.startsWith("pack-") //$NON-NLS-1$
+				|| !pf.getPackExt().equals(PACK)) {
+			throw new IOException(
+					MessageFormat.format(JGitText.get().notAValidPack, pack));
+		}
+
+		PackFile bitmapIdx = pf.create(BITMAP_INDEX);
+		Pack res = new Pack(pack, bitmapIdx.exists() ? bitmapIdx : null);
 		packed.insert(res);
 		return res;
 	}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/Pack.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/Pack.java
index fa938b3..5efd4c5 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/Pack.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/Pack.java
@@ -12,7 +12,6 @@
 
 package org.eclipse.jgit.internal.storage.file;
 
-import static org.eclipse.jgit.internal.storage.pack.PackExt.BITMAP_INDEX;
 import static org.eclipse.jgit.internal.storage.pack.PackExt.INDEX;
 import static org.eclipse.jgit.internal.storage.pack.PackExt.KEEP;
 
@@ -38,6 +37,7 @@
 import java.util.zip.DataFormatException;
 import java.util.zip.Inflater;
 
+import org.eclipse.jgit.annotations.Nullable;
 import org.eclipse.jgit.errors.CorruptObjectException;
 import org.eclipse.jgit.errors.LargeObjectException;
 import org.eclipse.jgit.errors.MissingObjectException;
@@ -51,7 +51,6 @@
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.internal.storage.pack.BinaryDelta;
 import org.eclipse.jgit.internal.storage.pack.ObjectToPack;
-import org.eclipse.jgit.internal.storage.pack.PackExt;
 import org.eclipse.jgit.internal.storage.pack.PackOutputStream;
 import org.eclipse.jgit.lib.AbbreviatedObjectId;
 import org.eclipse.jgit.lib.AnyObjectId;
@@ -80,8 +79,6 @@
 
 	private final PackFile packFile;
 
-	private final int extensions;
-
 	private PackFile keepFile;
 
 	final int hash;
@@ -105,7 +102,8 @@
 
 	private volatile Exception invalidatingCause;
 
-	private boolean invalidBitmap;
+	@Nullable
+	private PackFile bitmapIdxFile;
 
 	private AtomicInteger transientErrorCount = new AtomicInteger();
 
@@ -131,14 +129,14 @@
 	 *
 	 * @param packFile
 	 *            path of the <code>.pack</code> file holding the data.
-	 * @param extensions
-	 *            additional pack file extensions with the same base as the pack
+	 * @param bitmapIdxFile
+	 *            existing bitmap index file with the same base as the pack
 	 */
-	public Pack(File packFile, int extensions) {
+	public Pack(File packFile, @Nullable PackFile bitmapIdxFile) {
 		this.packFile = new PackFile(packFile);
 		this.fileSnapshot = PackFileSnapshot.save(packFile);
 		this.packLastModified = fileSnapshot.lastModifiedInstant();
-		this.extensions = extensions;
+		this.bitmapIdxFile = bitmapIdxFile;
 
 		// Multiply by 31 here so we can more directly combine with another
 		// value in WindowCache.hash(), without doing the multiply there.
@@ -1124,26 +1122,28 @@
 	}
 
 	synchronized PackBitmapIndex getBitmapIndex() throws IOException {
-		if (invalid || invalidBitmap)
+		if (invalid || bitmapIdxFile == null) {
 			return null;
-		if (bitmapIdx == null && hasExt(BITMAP_INDEX)) {
+		}
+		if (bitmapIdx == null) {
 			final PackBitmapIndex idx;
 			try {
-				idx = PackBitmapIndex.open(packFile.create(BITMAP_INDEX), idx(),
+				idx = PackBitmapIndex.open(bitmapIdxFile, idx(),
 						getReverseIdx());
 			} catch (FileNotFoundException e) {
 				// Once upon a time this bitmap file existed. Now it
 				// has been removed. Most likely an external gc  has
 				// removed this packfile and the bitmap
-				 invalidBitmap = true;
-				 return null;
+				bitmapIdxFile = null;
+				return null;
 			}
 
 			// At this point, idx() will have set packChecksum.
-			if (Arrays.equals(packChecksum, idx.packChecksum))
+			if (Arrays.equals(packChecksum, idx.packChecksum)) {
 				bitmapIdx = idx;
-			else
-				invalidBitmap = true;
+			} else {
+				bitmapIdxFile = null;
+			}
 		}
 		return bitmapIdx;
 	}
@@ -1179,10 +1179,6 @@
 		}
 	}
 
-	private boolean hasExt(PackExt ext) {
-		return (extensions & ext.getBit()) != 0;
-	}
-
 	@SuppressWarnings("nls")
 	@Override
 	public String toString() {
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackDirectory.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackDirectory.java
index 2e68d46..007205e 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackDirectory.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackDirectory.java
@@ -421,10 +421,7 @@
 				continue;
 			}
 
-			list.add(new Pack(packFile,
-					packFilesByExt.containsKey(BITMAP_INDEX)
-							? BITMAP_INDEX.getBit()
-							: 0));
+			list.add(new Pack(packFile, packFilesByExt.get(BITMAP_INDEX)));
 			foundNew = true;
 		}