Attached file is renamed from original name to digits like "459345" without extension

Describe the bug
I'm composing new mail and when I'm attaching a document or program file, for example "mydrawing.pdf", it is attaching like "123456" instead "mydrawing.pdf". Receiving this letter on PC I see "123456" as attachment and need to rename it back to *.pdf file. Then file opens by application properly.
txt/xls/doc/docx/pdf/apk/xapk/fb2/opf/zip/ - attached with renaming
jpg/png/mp3/mp4/ts - attached as is without renaming.
Other types isn't tested, I see media files attached correctly, documents/programs/archives - not.

To Reproduce
Standard procedure to create new mail: run K9 -> New mail - Attach a non-graphic/audio/video file -> Choose file -> Attach

Screenshots

Environment (please complete the following information):

• K-9 Mail version: 5.904
• Android version: 7.0
• Device: Honor 6X (4 RAM / 64 Gb disk)
• Account type: POP3
• No antiviruses

Additional context
This problem exists about 2-3 years

Logs
Please take some time to retrieve logs and attach them here:
k9-log.txt

Start by reproducing the reported attachment flow on Android 7.0 with a document or archive, using the supplied log for context. Trace the attachment handling from file selection and compare it with media files; done means non-media attachments retain their original names and extensions when received.