Added ReadableFile#size()

main/filesystem-api/src/main/java/org/cryptomator/filesystem/ReadableFile.java

@@ -29,8 +29,16 @@ public interface ReadableFile extends ReadableByteChannel {
 	 *             if an {@link IOException} occurs while reading from this
 	 *             {@code ReadableBytes}
 	 */
+	@Override
 	int read(ByteBuffer target) throws UncheckedIOException;
 
+	/**
+	 * @return The current size of the file. This value is a snapshot and might have been changed by concurrent modifications.
+	 * @throws UncheckedIOException
+	 *             if an {@link IOException} occurs
+	 */
+	long size() throws UncheckedIOException;
+
 	/**
 	 * <p>
 	 * Fast-forwards or rewinds the file to the specified position.

main/filesystem-api/src/main/java/org/cryptomator/filesystem/delegating/DelegatingReadableFile.java

@@ -42,6 +42,11 @@ public class DelegatingReadableFile implements ReadableFile {
 		return delegate.read(target);
 	}
 
+	@Override
+	public long size() throws UncheckedIOException {
+		return delegate.size();
+	}
+
 	@Override
 	public void position(long position) throws UncheckedIOException {
 		delegate.position(position);