[feature] Media attachment placeholders (#2331)

* [feature] Use placeholders for unknown media types

* fix read of underreported small files

* switch to reduce nesting

* simplify cleanup

internal/media/util.go

@@ -0,0 +1,42 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package media
+
+// newHdrBuf returns a buffer of suitable size to
+// read bytes from a file header or magic number.
+//
+// File header is *USUALLY* 261 bytes at the start
+// of a file; magic number can be much less than
+// that (just a few bytes).
+//
+// To cover both cases, this function returns a buffer
+// suitable for whichever is smallest: the first 261
+// bytes of the file, or the whole file.
+//
+// See:
+//
+//   - https://en.wikipedia.org/wiki/File_format#File_header
+//   - https://github.com/h2non/filetype.
+func newHdrBuf(fileSize int) []byte {
+	bufSize := 261
+	if fileSize > 0 && fileSize < bufSize {
+		bufSize = fileSize
+	}
+
+	return make([]byte, bufSize)
+}