fix(uploads): say a refused text upload is not UTF-8 instead of a type mismatch

An Excel or Notepad CSV/TXT export (UTF-16 with a BOM, or Windows-1252) was refused as "File bytes do not match declared type", which reads like a corrupt file. Name the encoding and tell the user to save it as UTF-8.

docs/api-reference.md

@@ -624,7 +624,9 @@ or `.csv` is unreliable): the office formats `.pdf`, `.docx`, `.pptx`, `.xlsx`,
 `.zip` and the text family `.txt`, `.md`, `.csv`, `.tsv`, `.json`, `.log`,
 `.yaml`, `.yml`, `.xml`. A document is buffered under the same 50 MB image cap,
 its bytes are checked against the extension (a magic-number signature, or UTF-8
-with no NUL byte for text) and answered 415 on a mismatch, and it is saved under
+with no NUL byte for text) and answered 415 on a mismatch (for text, an `error`
+saying the file is not UTF-8 text, or UTF-16 text when it opens with a UTF-16
+byte order mark, and to save it as UTF-8), and it is saved under
 a sanitized stem of its name, so `Q3 report.pdf` is findable as
 `paste-<ms>-<hex>-Q3-report.pdf`. A named type that is neither an image nor a
 document (a script, an HTML page, a `.sh`) is refused 400 `Unsupported file

docs/wiki/Working-With-Files.md

@@ -184,10 +184,12 @@ through server memory. Documents are the office formats (pdf, docx, pptx, xlsx,
 text family (txt, md, csv, tsv, json, log, yaml, yml, xml). Each is checked on the way in,
 against its file signature for the binary formats and against its encoding for text (UTF-8,
 no NUL bytes), so a script renamed to `.pdf` is refused while a `.txt` can hold any text at
-all. Files with script or page extensions (sh, py, js, html) are not accepted, since a prompt
-upload is something for the agent to read, not something for Codeman to store and serve. A
-document keeps a trimmed copy of its original name, letters, digits, dot, dash and underscore
-with anything else turned into a dash, so `Q3 report.pdf` is findable as
+all. A text file in another encoding is refused with a message saying it is not UTF-8 (or that
+it is UTF-16, which Excel's "Unicode Text" and Notepad's "Unicode" produce); save it as UTF-8
+(in Excel, "CSV UTF-8") and attach it again. Files with script or page extensions (sh, py,
+js, html) are not accepted, since a prompt upload is something for the agent to read, not
+something for Codeman to store and serve. A document keeps a trimmed copy of its original
+name, letters, digits, dot, dash and underscore with anything else turned into a dash, so `Q3 report.pdf` is findable as
 `paste-<time>-<id>-Q3-report.pdf`; a name made entirely of other characters saves without
 one. Uploads are refused for remote (SSH) sessions, whose working directory is on the other
 host.