feat(mux): add KnownFSTypes to list filesystem prefixes

this makes it possible to iterate a common set of supported prefix types. Exporting this string
slice as the return value of a function gives us the opportunity to declare an interface for
modifying prefixes registered with the muxfs packaage in the future

adder.go

@@ -7,7 +7,7 @@ import (
 
 // ErrNotAddingFS is the canonical error to return when the AddingFS extension
 // interface is required but not available
-var ErrNotAddingFS = errors.New("This filesystem doesn't support batched adding")
+var ErrNotAddingFS = errors.New("this filesystem doesn't support batched adding")
 
 // AddedFile reports on the results of adding a file to the store
 type AddedFile struct {

fs.go

@@ -17,10 +17,6 @@ var (
 	ErrReadOnly = errors.New("readonly filesystem")
 )
 
-// func init() {
-// 	logger.SetLogLevel("qfs", "debug")
-// }
-
 // PathResolver is the "get" portion of a Filesystem
 type PathResolver interface {
 	Get(ctx context.Context, path string) (File, error)

muxfs/mux.go

@@ -62,11 +62,21 @@ func (m *Mux) Filesystem(fsType string) qfs.Filesystem {
 	return m.handlers[fsType]
 }
 
+// KnownFSTypes gives the set of filesystems known to muxfs.New
+func KnownFSTypes() []string {
+	return []string{
+		httpfs.FilestoreType,
+		qipfs.FilestoreType,
+		localfs.FilestoreType,
+		qfs.MemFilestoreType,
+	}
+}
+
 // constructors maps filesystem type strings to constructor functions
 var constructors = map[string]qfs.Constructor{
+	httpfs.FilestoreType:  httpfs.NewFilesystem,
 	qipfs.FilestoreType:   qipfs.NewFilesystem,
 	localfs.FilestoreType: localfs.NewFilesystem,
-	httpfs.FilestoreType:  httpfs.NewFilesystem,
 	qfs.MemFilestoreType:  qfs.NewMemFilesystem,
 }