Allow specifying/retaining names of downloaded files. (#488)

* Add support for specifying a filename when downloading files

* Document how to retain names of downloaded files

* Compare with `undefined` instead of using `typeof`

docs/downloading-files.md

@@ -1,7 +1,8 @@
 # File Downloading
 
 WeTTY supports file downloads by printing terminal escape sequences between a
-base64 encoded file.
+base64 encoded file. The name of the downloaded file can optionally be provided,
+also base64 encoded, before the encoded file contents with a `:` separating them.
 
 The terminal escape sequences used are `^[[5i` and `^[[4i` (VT100 for "enter
 auto print" and "exit auto print" respectively -
@@ -13,8 +14,14 @@ To take advantage add the following bash function to your `.bashrc`
 function wetty-download() {
   file=${1:-/dev/stdin}
 
-  if [[ -f $file || $file == "/dev/stdin" ]]; then
-    printf "\033[5i"$(cat $file | base64 -w 0)"\033[4i"
+  nameprefix=""
+  if [[ -f "$file" ]]; then
+    nameprefix="$(basename "$file" | base64 -w 0):"
+  fi
+
+
+  if [[ -f "$file" || "$file" == "/dev/stdin" ]]; then
+    printf "\033[5i"$nameprefix$(cat "$file" | base64 -w 0)"\033[4i"
   else
     echo "$file does not appear to be a file"
   fi
@@ -27,7 +34,7 @@ You are then able to download files via WeTTY!
 wetty-download my-pdf-file.pdf
 ```
 
-or you can still use the classic style: 
+or you can still use the classic style:
 
 ```bash
 $ cat my-pdf-file.pdf | wetty-download

src/client/wetty/download.ts

@@ -7,7 +7,11 @@ const DEFAULT_FILE_END = '\u001b[4i';
 type OnCompleteFile = (bufferCharacters: string) => void;
 
 function onCompleteFile(bufferCharacters: string): void {
+  let fileNameBase64;
   let fileCharacters = bufferCharacters;
+  if (bufferCharacters.includes(":")) {
+    [fileNameBase64, fileCharacters] = bufferCharacters.split(":");
+  }
   // Try to decode it as base64, if it fails we assume it's not base64
   try {
     fileCharacters = window.atob(fileCharacters);
@@ -34,12 +38,23 @@ function onCompleteFile(bufferCharacters: string): void {
     mimeType = 'text/plain';
     fileExt = 'txt';
   }
-  const fileName = `file-${new Date()
-    .toISOString()
-    .split('.')[0]
-    .replace(/-/g, '')
-    .replace('T', '')
-    .replace(/:/g, '')}${fileExt ? `.${fileExt}` : ''}`;
+  let fileName;
+  try {
+    if (fileNameBase64 !== undefined) {
+      fileName = window.atob(fileNameBase64);
+    }
+  } catch (err) {
+    // Filename wasn't base64-encoded so let's ignore it
+  }
+
+  if (fileName === undefined) {
+    fileName = `file-${new Date()
+      .toISOString()
+      .split('.')[0]
+      .replace(/-/g, '')
+      .replace('T', '')
+      .replace(/:/g, '')}${fileExt ? `.${fileExt}` : ''}`;
+  }
 
   const blob = new Blob([new Uint8Array(bytes.buffer)], {
     type: mimeType,