Revert "MM-27312 Allow uploading 10 files per post (#16235)" (#16908)

This reverts commit 8e2986e5ed.

model/post.go

@@ -45,8 +45,7 @@ const (
 	POST_EPHEMERAL              = "system_ephemeral"
 	POST_CHANGE_CHANNEL_PRIVACY = "system_change_chan_privacy"
 	POST_ADD_BOT_TEAMS_CHANNELS = "add_bot_teams_channels"
-	POST_MAX_FILES              = 10
-	POST_FILEIDS_MAX_RUNES      = 300
+	POST_FILEIDS_MAX_RUNES      = 150
 	POST_FILENAMES_MAX_RUNES    = 4000
 	POST_HASHTAGS_MAX_RUNES     = 1000
 	POST_MESSAGE_MAX_RUNES_V1   = 4000
@@ -340,12 +339,8 @@ func (o *Post) IsValid(maxPostSize int) *AppError {
 		return NewAppError("Post.IsValid", "model.post.is_valid.filenames.app_error", nil, "id="+o.Id, http.StatusBadRequest)
 	}
 
-	if len(o.FileIds) > POST_MAX_FILES {
-		return NewAppError("Post.IsValid", "model.post.is_valid.file_ids.app_error", nil, "id="+o.Id, http.StatusBadRequest)
-	}
-
 	if utf8.RuneCountInString(ArrayToJson(o.FileIds)) > POST_FILEIDS_MAX_RUNES {
-		return NewAppError("Post.IsValid", "model.post.is_valid.file_ids.app_error", nil, "id="+o.Id, http.StatusInternalServerError)
+		return NewAppError("Post.IsValid", "model.post.is_valid.file_ids.app_error", nil, "id="+o.Id, http.StatusBadRequest)
 	}
 
 	if utf8.RuneCountInString(StringInterfaceToJson(o.GetProps())) > POST_PROPS_MAX_RUNES {