[outtmpl] Alternate form for `D` and fix suffix's case

Fixes: https://github.com/yt-dlp/yt-dlp/issues/2085#issuecomment-1002247689, https://github.com/yt-dlp/yt-dlp/pull/2132/files#r775729811

README.md

@@ -1090,7 +1090,7 @@ The field names themselves (the part inside the parenthesis) can also have some
 
 1. **Default**: A literal default value can be specified for when the field is empty using a `|` separator. This overrides `--output-na-template`. Eg: `%(uploader|Unknown)s`
 
-1. **More Conversions**: In addition to the normal format types `diouxXeEfFgGcrs`, `B`, `j`, `l`, `q`, `D`, `S` can be used for converting to **B**ytes, **j**son (flag `#` for pretty-printing), a comma separated **l**ist (flag `#` for `\n` newline-separated), a string **q**uoted for the terminal (flag `#` to split a list into different arguments), to add **D**ecimal suffixes (Eg: 10M), and to **S**anitize as filename (flag `#` for restricted), respectively
+1. **More Conversions**: In addition to the normal format types `diouxXeEfFgGcrs`, `B`, `j`, `l`, `q`, `D`, `S` can be used for converting to **B**ytes, **j**son (flag `#` for pretty-printing), a comma separated **l**ist (flag `#` for `\n` newline-separated), a string **q**uoted for the terminal (flag `#` to split a list into different arguments), to add **D**ecimal suffixes (Eg: 10M) (flag `#` to use 1024 as factor), and to **S**anitize as filename (flag `#` for restricted), respectively
 
 1. **Unicode normalization**: The format type `U` can be used for NFC [unicode normalization](https://docs.python.org/3/library/unicodedata.html#unicodedata.normalize). The alternate form flag (`#`) changes the normalization to NFD and the conversion flag `+` can be used for NFKC/NFKD compatibility equivalence normalization. Eg: `%(title)+.100U` is NFKC
 

test/test_YoutubeDL.py

@@ -645,6 +645,7 @@ class TestYoutubeDL(unittest.TestCase):
         'ext': 'mp4',
         'width': None,
         'height': 1080,
+        'filesize': 1024,
         'title1': '$PATH',
         'title2': '%PATH%',
         'title3': 'foo/bar\\test',
@@ -778,8 +779,9 @@ class TestYoutubeDL(unittest.TestCase):
         test('%(title5)#U', 'a\u0301e\u0301i\u0301 𝐀')
         test('%(title5)+U', 'áéí A')
         test('%(title5)+#U', 'a\u0301e\u0301i\u0301 A')
-        test('%(height)D', '1K')
-        test('%(height)5.2D', ' 1.08K')
+        test('%(height)D', '1k')
+        test('%(filesize)#D', '1Ki')
+        test('%(height)5.2D', ' 1.08k')
         test('%(title4)#S', 'foo_bar_test')
         test('%(title4).10S', ('foo \'bar\' ', 'foo \'bar\'' + ('#' if compat_os_name == 'nt' else ' ')))
         if compat_os_name == 'nt':

yt_dlp/YoutubeDL.py

@@ -1166,7 +1166,9 @@ class YoutubeDL(object):
                     'NF%s%s' % ('K' if '+' in flags else '', 'D' if '#' in flags else 'C'),
                     value), str_fmt
             elif fmt[-1] == 'D':  # decimal suffix
-                value, fmt = format_decimal_suffix(value, f'%{fmt[:-1]}f%s' if fmt[:-1] else '%d%s'), 's'
+                num_fmt, fmt = fmt[:-1].replace('#', ''), 's'
+                value = format_decimal_suffix(value, f'%{num_fmt}f%s' if num_fmt else '%d%s',
+                                              factor=1024 if '#' in flags else 1000)
             elif fmt[-1] == 'S':  # filename sanitization
                 value, fmt = filename_sanitizer(initial_field, value, restricted='#' in flags), str_fmt
             elif fmt[-1] == 'c':

yt_dlp/utils.py

@@ -2116,9 +2116,11 @@ def format_decimal_suffix(num, fmt='%d%s', *, factor=1000):
     if num is None:
         return None
     exponent = 0 if num == 0 else int(math.log(num, factor))
-    suffix = ['', *'KMGTPEZY'][exponent]
+    suffix = ['', *'kMGTPEZY'][exponent]
+    if factor == 1024:
+        suffix = {'k': 'Ki', '': ''}.get(suffix, f'{suffix}i')
     converted = num / (factor ** exponent)
-    return fmt % (converted, f'{suffix}i' if suffix and factor == 1024 else suffix)
+    return fmt % (converted, suffix)
 
 
 def format_bytes(bytes):