Added the case to detect PDF format from file signature

SDWebImage/Core/NSData+ImageContentType.m

@@ -65,6 +65,15 @@
             }
             break;
         }
+        case 0x25: {
+            if (data.length >= 4) {
+                //%PDF
+                NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(1, 4)] encoding:NSASCIIStringEncoding];
+                if ([testString isEqualToString:@"PDF"]) {
+                    return SDImageFormatPDF;
+                }
+            }
+        }
     }
     return SDImageFormatUndefined;
 }