Recognize "Probable" embedded images

We use a pretty narrow definition, because we don't want to have
false-positives.  This change adds $F1 to the set of possible
ProDOS file types (which was previously just BIN and NON), and
accepts files with an unrecognized filename extension if their
length is exactly 140kiB.

(issue #35)

AppCommon/FileIdentifier.cs

@@ -123,6 +123,7 @@ public static bool HasDiskImageAttribs(IFileEntry entry, string gzipName, out st
             if (!looksGood && !hasProType && entry.HasProDOSTypes) {
                 proType = entry.FileType;
                 proAux = entry.AuxType;
+                hasProType = true;
             }
             if (!looksGood) {
                 if (proType == FileAttribs.FILE_TYPE_LBR &&
@@ -145,9 +146,16 @@ public static bool HasDiskImageAttribs(IFileEntry entry, string gzipName, out st
                 // If it's BIN or NON, or totally typeless (ZIP), test the filename extension.
                 if ((!hasProType /*&& !entry.HasHFSTypes*/) ||
                         (hasProType && (proType == FileAttribs.FILE_TYPE_NON ||
-                                        proType == FileAttribs.FILE_TYPE_BIN))) {
+                                        proType == FileAttribs.FILE_TYPE_BIN ||
+                                        proType == FileAttribs.FILE_TYPE_F1))) {
                     if (ExtInSet(ext, sDiskExts)) {
                         looksGood = true;
+                    } else if (entry.DataLength == 140 * 1024) {
+                        // File is exactly the size of a 140KB floppy disk image.  It didn't have
+                        // a recognized disk image filename extension, so let's assume it's in
+                        // DOS sector order.
+                        ext = ".do";
+                        looksGood = true;
                     }
                 }
             }

DiskArc/DAExtensions.cs

@@ -747,7 +747,7 @@ public static bool IsMacZipHeader(this IFileEntry entry) {
             return false;
         }
 
-        #endregion
+        #endregion IFileEntry
 
         #region IMetadata
 
@@ -768,6 +768,6 @@ public static bool IsMacZipHeader(this IFileEntry entry) {
             return null;
         }
 
-        #endregion
+        #endregion IMetadata
     }
 }

DiskArc/FileAttribs.cs

@@ -69,6 +69,7 @@ public sealed class FileAttribs {
         public const int FILE_TYPE_SND = 0xd8;
         public const int FILE_TYPE_LBR = 0xe0;
         public const int FILE_TYPE_CMD = 0xf0;
+        public const int FILE_TYPE_F1 = 0xf1;
         public const int FILE_TYPE_F2 = 0xf2;
         public const int FILE_TYPE_F3 = 0xf3;
         public const int FILE_TYPE_F4 = 0xf4;