Mime Multipart Builder
Build complete MIME text with CRLF and an explicit closing boundary. mixed/alternative/related use UTF-8 base64 bodies; form-data keeps UTF-8 text with CRLF normalization and percent-encodes non-ASCII filenames, not filename*. Reject header controls and boundary collisions. No attachments are read or sent. 32 KiB JSON / 50 parts / 70-character boundary.
Enabling local controls…
Results
Review the settings and run. Editing inputs clears old results.
Processing model and limits
Build complete MIME text with CRLF and an explicit closing boundary. mixed/alternative/related use UTF-8 base64 bodies; form-data keeps UTF-8 text with CRLF normalization and percent-encodes non-ASCII filenames, not filename*. Reject header controls and boundary collisions. No attachments are read or sent. 32 KiB JSON / 50 parts / 70-character boundary.
Data and model checked: RFC 7578 multipart form data ↗Source tool controls ↗
How to use this tool
Load the example, confirm field units and processing rules, then run locally. Editing any input clears the old result. Copy or download the complete output explicitly.
When not to use it
Build complete MIME text with CRLF and an explicit closing boundary. mixed/alternative/related use UTF-8 base64 bodies; form-data keeps UTF-8 text with CRLF normalization and percent-encodes non-ASCII filenames, not filename*. Reject header controls and boundary collisions. No attachments are read or sent. 32 KiB JSON / 50 parts / 70-character boundary.