In writing my new Outlook Add-in (Send to Trello), I got stuck on attachments. The first version did not include an attachments option because of two unique problems that compounded each other:
- The Office Add-in API no longer provides a Media Type/MIME Type with an attachment request. I am able to get the “blob()” from Office, but other than the file extension there is not a way to determine the type. But sometimes a file does not have an extension, or the extension is wrong, etc.
- The Trello API will not let you upload without supplying a MIME type, you cannot just give it a Base64 string as an attachment and let them figure it out.
So, I found out something interesting while researching a workaround. Most every base64 string of a specific file type starts with the same “prolog” of text. Using this, combined with the fallback of the file extensions, I was able to get attachments to work (for the attachment types supported by Trello). So, v1.02 will now include attachments.
Anway, as for the workaround I found, this might be ugly, but wanted to share it anyway:
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| /** | |
| * Returns the data type based on the base64 string | |
| * @param {String} base64String | |
| * @param {String} fileName | |
| * @returns {String} | |
| */ | |
| detectMimeType(base64String, fileName) { | |
| var ext = fileName.substring(fileName.lastIndexOf(".") + 1); | |
| if (ext === undefined || ext === null || ext === "") ext = "bin"; | |
| ext = ext.toLowerCase(); | |
| const signatures = { | |
| JVBERi0: "application/pdf", | |
| R0lGODdh: "image/gif", | |
| R0lGODlh: "image/gif", | |
| iVBORw0KGgo: "image/png", | |
| TU0AK: "image/tiff", | |
| "/9j/": "image/jpg", | |
| UEs: "application/vnd.openxmlformats-officedocument.", | |
| PK: "application/zip", | |
| }; | |
| for (var s in signatures) { | |
| if (base64String.indexOf(s) === 0) { | |
| var x = signatures[s]; | |
| // if an office file format | |
| if (ext.length > 3 && ext.substring(0, 3) === "ppt") { | |
| x += "presentationml.presentation"; | |
| } else if (ext.length > 3 && ext.substring(0, 3) === "xls") { | |
| x += "spreadsheetml.sheet"; | |
| } else if (ext.length > 3 && ext.substring(0, 3) === "doc") { | |
| x += "wordprocessingml.document"; | |
| } | |
| // return | |
| return x; | |
| } | |
| } | |
| // if we are here we can only go off the extensions | |
| const extensions = { | |
| xls: "application/vnd.ms-excel", | |
| ppt: "application/vnd.ms-powerpoint", | |
| doc: "application/msword", | |
| xml: "text/xml", | |
| mpeg: "audio/mpeg", | |
| mpg: "audio/mpeg", | |
| txt: "text/plain", | |
| }; | |
| for (var e in extensions) { | |
| if (ext.indexOf(e) === 0) { | |
| var xx = extensions[e]; | |
| return xx; | |
| } | |
| } | |
| // if we are here – not sure what type this is | |
| return "unknown"; | |
| } |
Nice Code,
Here are a free online File mime type checker tool.