[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/api/model/attachment.go

@@ -70,10 +70,10 @@ type Attachment struct {
 	URL *string `json:"url"`
 	// A shorter URL for the attachment.
 	// In our case, we just give the URL again since we don't create smaller URLs.
-	TextURL string `json:"text_url"`
+	TextURL *string `json:"text_url"`
 	// The location of a scaled-down preview of the attachment.
 	// example: https://example.org/fileserver/some_id/attachments/some_id/small/attachment.jpeg
-	PreviewURL string `json:"preview_url"`
+	PreviewURL *string `json:"preview_url"`
 	// The location of the full-size original attachment on the remote server.
 	// Only defined for instances other than our own.
 	// example: https://some-other-server.org/attachments/original/ahhhhh.jpeg
@@ -83,13 +83,13 @@ type Attachment struct {
 	// example: https://some-other-server.org/attachments/small/ahhhhh.jpeg
 	PreviewRemoteURL *string `json:"preview_remote_url"`
 	// Metadata for this attachment.
-	Meta MediaMeta `json:"meta,omitempty"`
+	Meta *MediaMeta `json:"meta"`
 	// Alt text that describes what is in the media attachment.
 	// example: This is a picture of a kitten.
 	Description *string `json:"description"`
 	// A hash computed by the BlurHash algorithm, for generating colorful preview thumbnails when media has not been downloaded yet.
 	// See https://github.com/woltapp/blurhash
-	Blurhash string `json:"blurhash,omitempty"`
+	Blurhash *string `json:"blurhash"`
 }
 
 // MediaMeta models media metadata.