I know ArcGIS Desktop allows you to add any file type, whereas ArcGIS Online allows you to add specific file types as attachments.
Is the list of supported ArcGIS Online file types documented?
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List of allowed file types is published at the end of this ArcGIS online help page: https://doc.arcgis.com/en/arcgis-online/manage-data/work-with-tables.htm.
The following file formats are supported as attachments: 7Z, AIF, AVI, BMP, DOC, DOCX, DOT, ECW, EMF, EPA, GIF, GML, GTAR, GZ, IMG, J2K, JP2, JPC, JPE, JPEG, JPF, JPG, JSON, MDB, MID, MOV, MP2, MP3, MP4, MPA, MPE, MPEG, MPG, MPV2, PDF, PNG, PPT, PPTX, PS, PSD, QT, RA, RAM, RAW, RMI, SID, TAR, TGZ, TIF, TIFF, TXT, VRML, WAV, WMA, WMF, WPS, XLS, XLSX, XLT, XML, and ZIP.