Add support to fs.fs on serve static files (#1640)

* substitute *os.File by fs.File

* refactor error handling by using the new recommended form

* finish implementation

* substitute seek(offset,0) by seek(offset, io.SeekStart)

* add unit test

* use io.SeekStart on Seek method

compress.go

@@ -4,7 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
-	"os"
+	"io/fs"
 	"sync"
 
 	"github.com/klauspost/compress/flate"
@@ -421,7 +421,7 @@ func newCompressWriterPoolMap() []*sync.Pool {
 	return m
 }
 
-func isFileCompressible(f *os.File, minCompressRatio float64) bool {
+func isFileCompressible(f fs.File, minCompressRatio float64) bool {
 	// Try compressing the first 4kb of the file
 	// and see if it can be compressed by more than
 	// the given minCompressRatio.
@@ -433,7 +433,11 @@ func isFileCompressible(f *os.File, minCompressRatio float64) bool {
 	}
 	_, err := copyZeroAlloc(zw, lr)
 	releaseStacklessGzipWriter(zw, CompressDefaultCompression)
-	f.Seek(0, 0) //nolint:errcheck
+	seeker, ok := f.(io.Seeker)
+	if !ok {
+		return false
+	}
+	seeker.Seek(0, io.SeekStart) //nolint:errcheck
 	if err != nil {
 		return false
 	}