Saving Media

Last updated 2026-10-21

Each platform's Save method downloads a received file to a directory and returns the saved path.

Signatures

// core/telegram — fileID from Input.Photo[i].FileID or Input.Document.FileID
func (b *Bot) Save(ctx context.Context, fileID, dir string) (string, error)
// core/discord — one entry of Input.Attachments
func (b *Bot) Save(ctx context.Context, att *discordgo.MessageAttachment, dir string) (string, error)
// core/line — Input.MessageID of an image, video, audio, or file message
func (b *Bot) Save(ctx context.Context, messageID, dir string) (string, error)

Per-platform behavior

	Telegram	Discord	LINE
Size cap	20 MiB (Bot API `getFile` limit)	25 MiB	50 MiB (library limit)
Size known before download	`GetFile` → `FileSize`	`att.Size`	`ContentLength`
Download	`FileDownloadLink` over `http.DefaultClient`	`att.URL` over `http.DefaultClient`	`GetMessageContent` stream
Extension	From Telegram's `file_path`	From `att.Filename`	From the response `Content-Type`

Shared steps

Reject empty arguments (fileID / att / att.URL / messageID, and dir).
Reject the file when the reported size exceeds the cap, before downloading.
On a non-200 HTTP status (Telegram, Discord), return http <code>: <body> with at most 8 KiB of the body.
Create dir with os.MkdirAll(dir, 0755).
Stream into a temporary file <uuid>.*.tmp inside dir, reading at most cap + 1 bytes. If more than the cap arrives (the reported size was missing or wrong), delete the temp file and return an error. Oversized files are never truncated.
Rename the temp file to <uuid><ext> and return the full path.

The file name is a v4 UUID from go-pkg/utils; an empty UUID (random source failure) is returned as an error. The original Telegram/Discord file name is not kept; for LINE file messages the original name is in Input.FileName.

LINE extension mapping

image/jpeg → .jpg, image/png → .png, image/gif → .gif, image/webp → .webp, video/mp4 → .mp4, audio/mp4 and audio/x-m4a → .m4a, audio/aac → .aac, application/pdf → .pdf. Other types use the first extension from mime.ExtensionsByType; unknown types get no extension.

Example

bot.Reply(func(ctx context.Context, in discord.Input) string {
    for _, att := range in.Attachments {
        path, err := bot.Save(ctx, att, "./uploads")
        if err != nil {
            return "save failed: " + err.Error()
        }
        log.Println("saved", path)
    }
    return ""
})

Telegram Setup (Input.Photo, Input.Document)
Discord Setup (Input.Attachments)
LINE Messages (media message events)