allow spaces in file names

This will be more important as people work with media, since it is quite
common to use spaces in filenames generated by humans.  Media files will
not be built by fdroid, so most likely, they will have human-generated
names.

app/src/main/java/org/fdroid/fdroid/data/SanitizedFile.java

@@ -15,7 +15,7 @@ public class SanitizedFile extends File {
      * Removes anything that is not an alpha numeric character, or one of "-", ".", or "_".
      */
     public static String sanitizeFileName(String name) {
-        return name.replaceAll("[^A-Za-z0-9-._]", "");
+        return name.replaceAll("[^A-Za-z0-9-._ ]", "");
     }
 
     /**