diff --git a/src/encoding/bzip2.c b/src/encoding/bzip2.c
index 88c83bb4a..e3ddfe76d 100644
--- a/src/encoding/bzip2.c
+++ b/src/encoding/bzip2.c
@@ -179,9 +179,9 @@ bzip2_close(struct stream_encoded *stream)
 	}
 }
 
-static unsigned char *bzip2_extensions[] = { ".bz2", ".tbz", NULL };
+static const unsigned char *const bzip2_extensions[] = { ".bz2", ".tbz", NULL };
 
-struct decoding_backend bzip2_decoding_backend = {
+const struct decoding_backend bzip2_decoding_backend = {
 	"bzip2",
 	bzip2_extensions,
 	bzip2_open,
diff --git a/src/encoding/bzip2.h b/src/encoding/bzip2.h
index 2718b249c..1367500e7 100644
--- a/src/encoding/bzip2.h
+++ b/src/encoding/bzip2.h
@@ -4,7 +4,7 @@
 #include "encoding/encoding.h"
 
 #ifdef CONFIG_BZIP2
-extern struct decoding_backend bzip2_decoding_backend;
+extern const struct decoding_backend bzip2_decoding_backend;
 #else
 #define bzip2_decoding_backend dummy_decoding_backend
 #endif
diff --git a/src/encoding/encoding.c b/src/encoding/encoding.c
index 68158e25a..42dc62b16 100644
--- a/src/encoding/encoding.c
+++ b/src/encoding/encoding.c
@@ -69,9 +69,9 @@ dummy_close(struct stream_encoded *stream)
 	mem_free(stream->data);
 }
 
-static unsigned char *dummy_extensions[] = { NULL };
+static const unsigned char *const dummy_extensions[] = { NULL };
 
-static struct decoding_backend dummy_decoding_backend = {
+static const struct decoding_backend dummy_decoding_backend = {
 	"none",
 	dummy_extensions,
 	dummy_open,
@@ -87,7 +87,7 @@ static struct decoding_backend dummy_decoding_backend = {
 #include "encoding/gzip.h"
 #include "encoding/lzma.h"
 
-static struct decoding_backend *decoding_backends[] = {
+static const struct decoding_backend *const decoding_backends[] = {
 	&dummy_decoding_backend,
 	&gzip_decoding_backend,
 	&bzip2_decoding_backend,
@@ -147,7 +147,7 @@ close_encoded(struct stream_encoded *stream)
 
 
 /* Return a list of extensions associated with that encoding. */
-unsigned char **listext_encoded(enum stream_encoding encoding)
+const unsigned char *const *listext_encoded(enum stream_encoding encoding)
 {
 	return decoding_backends[encoding]->extensions;
 }
@@ -160,7 +160,7 @@ guess_encoding(unsigned char *filename)
 	int enc;
 
 	for (enc = 1; enc < ENCODINGS_KNOWN; enc++) {
-		unsigned char **ext = decoding_backends[enc]->extensions;
+		const unsigned char *const *ext = decoding_backends[enc]->extensions;
 
 		while (ext && *ext) {
 			int len = strlen(*ext);
@@ -175,7 +175,7 @@ guess_encoding(unsigned char *filename)
 	return ENCODING_NONE;
 }
 
-unsigned char *
+const unsigned char *
 get_encoding_name(enum stream_encoding encoding)
 {
 	return decoding_backends[encoding]->name;
@@ -194,7 +194,7 @@ try_encoding_extensions(struct string *filename, int *fd)
 
 	/* No file of that name was found, try some others names. */
 	for (encoding = 1; encoding < ENCODINGS_KNOWN; encoding++) {
-		unsigned char **ext = listext_encoded(encoding);
+		const unsigned char *const *ext = listext_encoded(encoding);
 
 		for (; ext && *ext; ext++) {
 			add_to_string(filename, *ext);
diff --git a/src/encoding/encoding.h b/src/encoding/encoding.h
index 2c0bfbae0..8b32c3b38 100644
--- a/src/encoding/encoding.h
+++ b/src/encoding/encoding.h
@@ -20,8 +20,8 @@ struct stream_encoded {
 };
 
 struct decoding_backend {
-	unsigned char *name;
-	unsigned char **extensions;
+	const unsigned char *name;
+	const unsigned char *const *extensions;
 	int (*open)(struct stream_encoded *stream, int fd);
 	int (*read)(struct stream_encoded *stream, unsigned char *data, int len);
 	unsigned char *(*decode_buffer)(unsigned char *data, int len, int *new_len);
@@ -33,9 +33,9 @@ int read_encoded(struct stream_encoded *, unsigned char *, int);
 unsigned char *decode_encoded_buffer(enum stream_encoding encoding, unsigned char *data, int len, int *new_len);
 void close_encoded(struct stream_encoded *);
 
-unsigned char **listext_encoded(enum stream_encoding);
+const unsigned char *const *listext_encoded(enum stream_encoding);
 enum stream_encoding guess_encoding(unsigned char *filename);
-unsigned char *get_encoding_name(enum stream_encoding encoding);
+const unsigned char *get_encoding_name(enum stream_encoding encoding);
 
 /* Read from open @stream into the @page string */
 enum connection_state
diff --git a/src/encoding/gzip.c b/src/encoding/gzip.c
index db469242b..a46800078 100644
--- a/src/encoding/gzip.c
+++ b/src/encoding/gzip.c
@@ -229,9 +229,9 @@ gzip_close(struct stream_encoded *stream)
 	gzclose((gzFile *) stream->data);
 }
 
-static unsigned char *gzip_extensions[] = { ".gz", ".tgz", NULL };
+static const unsigned char *const gzip_extensions[] = { ".gz", ".tgz", NULL };
 
-struct decoding_backend gzip_decoding_backend = {
+const struct decoding_backend gzip_decoding_backend = {
 	"gzip",
 	gzip_extensions,
 	gzip_open,
diff --git a/src/encoding/gzip.h b/src/encoding/gzip.h
index 7965a4d82..c13038e33 100644
--- a/src/encoding/gzip.h
+++ b/src/encoding/gzip.h
@@ -4,7 +4,7 @@
 #include "encoding/encoding.h"
 
 #ifdef CONFIG_GZIP
-extern struct decoding_backend gzip_decoding_backend;
+extern const struct decoding_backend gzip_decoding_backend;
 #else
 #define gzip_decoding_backend dummy_decoding_backend
 #endif
diff --git a/src/encoding/lzma.c b/src/encoding/lzma.c
index ad50a9e00..f4d134591 100644
--- a/src/encoding/lzma.c
+++ b/src/encoding/lzma.c
@@ -182,9 +182,9 @@ lzma_close(struct stream_encoded *stream)
 	lzma_cleanup(data);
 }
 
-static unsigned char *lzma_extensions[] = { ".lzma", NULL };
+static const unsigned char *const lzma_extensions[] = { ".lzma", NULL };
 
-struct decoding_backend lzma_decoding_backend = {
+const struct decoding_backend lzma_decoding_backend = {
 	"lzma",
 	lzma_extensions,
 	lzma_open,
diff --git a/src/encoding/lzma.h b/src/encoding/lzma.h
index 474d3dd38..9268aee1c 100644
--- a/src/encoding/lzma.h
+++ b/src/encoding/lzma.h
@@ -4,7 +4,7 @@
 #include "encoding/encoding.h"
 
 #ifdef CONFIG_LZMA
-extern struct decoding_backend lzma_decoding_backend;
+extern const struct decoding_backend lzma_decoding_backend;
 #else
 #define lzma_decoding_backend dummy_decoding_backend
 #endif
diff --git a/src/mime/mime.c b/src/mime/mime.c
index 5c39b43f7..f50dc9fc1 100644
--- a/src/mime/mime.c
+++ b/src/mime/mime.c
@@ -130,7 +130,7 @@ static inline unsigned char *
 check_encoding_type(unsigned char *extension)
 {
 	enum stream_encoding encoding = guess_encoding(extension);
-	unsigned char **extension_list;
+	const unsigned char *const *extension_list;
 	unsigned char *last_extension = strrchr(extension, '.');
 
 	if (encoding == ENCODING_NONE || !last_extension)