Initial work on improving docs.

docs/internals/uploading-files.md

@@ -0,0 +1,108 @@
+# Uploading Files
+
+To make files work as expected, there's a lot going on behind the scenes. Make
+sure to read through the [Files](../getting-started/files.md) section in
+Getting Started first as we'll be building on that information.
+
+This section only talks about file uploading. For non-uploaded files such as
+URLs and file IDs, you just need to pass a string.
+
+## Fields
+
+Let's start by talking about how the library represents files as part of a
+Config.
+
+### Static Fields
+
+Most endpoints use static file fields. For example, `sendPhoto` expects a single
+file named `photo`. All we have to do is set that single field with the correct
+value (either a string or multipart file). Methods like `sendDocument` take two
+file uploads, a `document` and a `thumb`. These are pretty straightforward.
+
+Remembering that the `Fileable` interface only requires one method, let's
+implement it for `DocumentConfig`.
+
+```go
+func (config DocumentConfig) files() []RequestFile {
+    // We can have multiple files, so we'll create an array. We also know that
+    // there always is a document file, so initialize the array with that.
+	files := []RequestFile{{
+		Name: "document",
+		File: config.File,
+	}}
+
+    // We'll only add a file if we have one.
+	if config.Thumb != nil {
+		files = append(files, RequestFile{
+			Name: "thumb",
+			File: config.Thumb,
+		})
+	}
+
+	return files
+}
+```
+
+Telegram also supports the `attach://` syntax (discussed more later) for
+thumbnails, but there's no reason to make things more complicated.
+
+### Dynamic Fields
+
+Of course, not everything can be so simple. Methods like `sendMediaGroup`
+can accept many files, and each file can have custom markup. Using a static
+field isn't possible because we need to specify which field is attached to each
+item. Telegram introduced the `attach://` syntax for this.
+
+Let's follow through creating a new media group with string and file uploads.
+
+First, we start by creating some `InputMediaPhoto`.
+
+```go
+photo := tgbotapi.NewInputMediaPhoto("tests/image.jpg")
+url := tgbotapi.NewInputMediaPhoto(tgbotapi.FileURL("https://i.imgur.com/unQLJIb.jpg"))
+```
+
+This created a new `InputMediaPhoto` struct, with a type of `photo` and the
+media interface that we specified.
+
+We'll now create our media group with the photo and URL.
+
+```go
+mediaGroup := NewMediaGroup(ChatID, []interface{}{
+    photo,
+    url,
+})
+```
+
+A `MediaGroupConfig` stores all of the media in an array of interfaces. We now
+have all of the data we need to upload, but how do we figure out field names for
+uploads? We didn't specify `attach://unique-file` anywhere.
+
+When the library goes to upload the files, it looks at the `params` and `files`
+for the Config. The params are generated by transforming the file into a value
+more suitable for uploading, file IDs and URLs are untouched but uploaded types
+are all changed into `attach://file-%d`. When collecting a list of files to
+upload, it names them the same way. This creates a nearly transparent way of
+handling multiple files in the background without the user having to consider
+what's going on.
+
+## Library Processing
+
+If at some point in the future new upload types are required, let's talk about
+where the current types are used.
+
+Upload types are defined in `configs.go`. Where possible, type aliases are
+preferred. Structs can be used when multiple fields are required.
+
+The main usage of the upload types happens in `UploadFiles`. It switches on each
+file's type in order to determine how to upload it. Files that aren't uploaded
+(file IDs, URLs) are converted back into strings and passed through as strings
+into the correct field. Uploaded types are processed as needed (opening files,
+etc.) and written into the form using a copy approach in a goroutine to reduce
+memory usage.
+
+In addition to `UploadFiles`, there's more processing of upload types in the
+`prepareInputMediaParam` and `prepareInputMediaFile` functions. These look at
+the `InputMedia` types to determine which files are uploaded and which are
+passed through as strings. They only need to be aware of which files need to be
+replaced with `attach://` fields.