Default format .spi for file-write is consistent

running e.g. mask_create with "--o test" produced a spider-format image
named "test", rather than "test.spi". This is now fixed.

src/filename.cpp

@@ -301,7 +301,7 @@ FileName FileName::getFileFormat() const
     else if ( (first = rfind("."))!=std::string::npos)
         result = substr(first + 1);
     else
-        result="spi";
+        result="";
     return result.toLowercase();
 
 }

src/image.h

@@ -132,6 +132,10 @@ class fImageHandler
     		closeFile();
 
     	FileName fileName, headName = "";
+    	// get the format, checking for possible format specifier before suffix
+    	// getFileFormat("file.spi")    	will return "spi"
+    	// getFileFormat("file.spi:mrc") 	will return "mrc"
+    	// getFileFormat("file") 			will return ""
 		ext_name = name.getFileFormat();
 
 		long int dump;
@@ -140,6 +144,8 @@ class fImageHandler
 		if (dump > 0)
 			dump--;
 
+		// create the filename from a possible input format specifier (file.spi:mrc means "it's called .spi, but it's really a .mrc")
+		// file.spi:mrc -> file.spi
 		fileName = fileName.removeFileFormat();
 
 		size_t found = fileName.find_first_of("%");
@@ -177,6 +183,11 @@ class fImageHandler
 			headName = fileName.addExtension("hed");
 			fileName = fileName.addExtension("img");
 		}
+		else if(ext_name=="")
+		{
+			ext_name="spi"; // SPIDER is default format if none is specified
+			fileName = fileName.addExtension(ext_name);
+		}
 
 		// Open image file
 		if ( ( fimg = fopen(fileName.c_str(), wmChar.c_str()) ) == NULL )